The effects of repeated corticosterone exposure on the interoceptive effects of alcohol in rats

mGlu2 and mGlu3 receptor negative allosteric modulators attenuate the interoceptive effects of alcohol in male and female rats

RATIONALE

The subjective effects of alcohol are associated with alcohol use disorder (AUD) vulnerability and treatment outcomes. The interoceptive effects of alcohol are part of these subjective effects and can be measured in animal models using drug discrimination procedures. The newly developed mGlu2 and mGlu3 negative allosteric modulators (NAMs) are potential therapeutics for AUD and may alter interoceptive sensitivity to alcohol.

OBJECTIVES

To determine the effects of mGlu2 and mGlu3 NAMs on the interoceptive effects of alcohol in rats.

METHODS

Long-Evans rats were trained to discriminate the interoceptive stimulus effects of alcohol (2.0 g/kg, i.g.) from water using both operant (males only) and Pavlovian (male and female) drug discrimination techniques. Following acquisition training, an alcohol dose-response (0, 0.5, 1.0, 2.0 g/kg) experiment was conducted to confirm stimulus control over behavior. Next, to test the involvement of mGlu2 and mGlu3, rats were pretreated with the mGlu2-NAM (VU6001966; 0, 3, 6, 12 mg/kg, i.p.) or the mGlu3-NAM (VU6010572; 0, 3, 6, 12 mg/kg, i.p.) before alcohol administration (2.0 g/kg, i.g.).

RESULTS

In Pavlovian discrimination, male rats showed greater interoceptive sensitivity to 1.0 and 2.0 g/kg alcohol compared to female rats. Both mGlu2-NAM and mGlu3-NAM attenuated the interoceptive effects of alcohol in male and female rats using Pavlovian and operant discrimination. There may be a potential sex difference in response to the mGlu2-NAM at the highest dose tested.

CONCLUSIONS

Male rats may be more sensitive to the interoceptive effects of the 2.0 g/kg alcohol training dose compared to female rats. Both mGlu2-and mGlu3-NAM attenuate the interoceptive effects of alcohol in male and female rats. These drugs may have potential for treatment of AUD in part by blunting the subjective effects of alcohol.

Sex-Specific Impact of Fkbp5 on Hippocampal Response to Acute Alcohol Injection: Involvement in Alterations of Metabolism-Related Pathways

High levels of alcohol intake alter brain gene expression and can produce long-lasting effects. FK506-binding protein 51 (FKBP51) encoded by Fkbp5 is a physical and cellular stress response gene and has been associated with alcohol consumption and withdrawal severity. Fkbp5 has been previously linked to neurite outgrowth and hippocampal morphology, sex differences in stress response, and epigenetic modification. Presently, primary cultured Fkbp5 KO and WT mouse neurons were examined for neurite outgrowth and mitochondrial signal with and without alcohol. We found neurite specification differences between KO and WT; particularly, mesh-like morphology was observed after alcohol treatment and confirmed higher MitoTracker signal in cultured neurons of Fkbp5 KO compared to WT at both naive and alcohol-treated conditions. Brain regions that express FKBP51 protein were identified, and hippocampus was confirmed to possess a high level of expression. RNA-seq profiling was performed using the hippocampus of naïve or alcohol-injected (2 mg EtOH/Kg) male and female Fkbp5 KO and WT mice. Differentially expressed genes (DEGs) were identified between Fkbp5 KO and WT at baseline and following alcohol treatment, with female comparisons possessing a higher number of DEGs than male comparisons. Pathway analysis suggested that genes affecting calcium signaling, lipid metabolism, and axon guidance were differentially expressed at naïve condition between KO and WT. Alcohol treatment significantly affected pathways and enzymes involved in biosynthesis (Keto, serine, and glycine) and signaling (dopamine and insulin receptor), and neuroprotective role. Functions related to cell morphology, cell-to-cell signaling, lipid metabolism, injury response, and post-translational modification were significantly altered due to alcohol. In summary, Fkbp5 plays a critical role in the response to acute alcohol treatment by altering metabolism and signaling-related genes.

Predator odor (TMT) exposure potentiates interoceptive sensitivity to alcohol and increases GABAergic gene expression in the anterior insular cortex and nucleus accumbens in male rats

Post-traumatic stress disorder (PTSD) confers enhanced vulnerability to developing comorbid alcohol use disorder (AUD). Exposure to the scent of a predator, such as the fox odor TMT, has been used to model a traumatic stressor with relevance to PTSD symptomatology. Alcohol produces distinct interoceptive (subjective) effects that may influence vulnerability to problem drinking and AUD. As such, understanding the lasting impact of stressors on sensitivity to the interoceptive effects of alcohol is clinically relevant. The present study used a 2-lever, operant drug discrimination procedure to train male Long-Evans rats to discriminate the interoceptive effects of alcohol (2 g/kg, i.g. [intragastrically]) from water. Upon stable performance, rats underwent a 15-min exposure to TMT. Two weeks later, an alcohol dose-response curve was conducted to evaluate the lasting effects of the TMT stressor on the interoceptive effects of alcohol. The TMT group showed a leftward shift in the effective dose (ED₅₀) of the dose-response curve compared to controls, reflecting potentiated interoceptive sensitivity to alcohol. TMT exposure did not affect response rate. GABAergic signaling in both the anterior insular cortex (aIC) and the nucleus accumbens (Acb) is involved in the interoceptive effects of alcohol and stressor-induced adaptations. As such, follow-up experiments in alcohol-naïve rats examined neuronal activation (as measured by c-Fos immunoreactivity) following TMT and showed that TMT exposure increased c-Fos expression in the aIC and the nucleus accumbens core (AcbC). Two weeks after TMT exposure, Gad-1 gene expression was elevated in the aIC and Gat-1 was increased in the Acb, compared to controls. Lastly, the alcohol discrimination and alcohol-naïve groups displayed dramatic differences in stress reactive behaviors during the TMT exposure, suggesting that alcohol exposure may alter the behavioral response to predator odor. Together, these data suggest that predator odor stressor results in potentiated sensitivity to alcohol, possibly through GABAergic adaptations in the aIC and Acb, which may be relevant to understanding PTSD-AUD comorbidity.

Early life stress and substance use disorders: The critical role of adolescent substance use

Early life stress (ELS) is a well-established risk factor for many psychiatric and medical disorders, including substance use disorders (SUDs). The relationship between ELS and SUDs is complex and there are likely multiple pathways from ELS to adverse substance use outcomes. The association between ELS and substance use emerges in adolescence. Adolescence is a critical period in development during which substance exposure markedly increases risk for SUDs. Therefore, this review focuses on the literature supporting the hypothesis that ELS increases risk for the development of SUDs through its influence on adolescent substance use. We discuss studies substantiating the role of ELS in adolescent substance use and explore how internalizing and externalizing psychopathology may be antecedents of substance use in adolescence. We examine clinical work suggesting ELS sculpts the Hypothalamic-Pituitary-Adrenal (HPA) Axis and developing brain-particularly subcortical brain regions that underlie stress response, mesocorticolimbic brain systems associated with reward sensitivity, and prefrontal regions that underlie executive control-in a way that increases risk for adolescent substance use and SUDs. We further explore how substance use during adolescence alters structure and function of these same systems, and how brain changes following ELS and adolescent substance use may independently, additively, or interactively contribute to risk for addiction. We conclude by discussing how the current literature can inform interventions aimed at reducing risk for SUDs in individuals with a history of ELS.

Interoception and alcohol: Mechanisms, networks, and implications

Interoception refers to the perception of the internal state of the body and is increasingly being recognized as an important factor in mental health disorders. Drugs of abuse produce powerful interoceptive states that are upstream of behaviors that drive and influence drug intake, and addiction pathology is impacted by interoceptive processes. The goal of the present review is to discuss interoceptive processes related to alcohol. We will cover physiological responses to alcohol, how interoceptive states can impact drinking, and the recruitment of brain networks as informed by clinical research. We also review the molecular and brain circuitry mechanisms of alcohol interoceptive effects as informed by preclinical studies. Finally, we will discuss emerging treatments with consideration of interoception processes. As our understanding of the role of interoception in drug and alcohol use grows, we suggest that the convergence of information provided by clinical and preclinical studies will be increasingly important. Given the complexity of interoceptive processing and the multitude of brain regions involved, an overarching network-based framework can provide context for how focused manipulations modulate interoceptive processing as a whole. In turn, preclinical studies can systematically determine the roles of individual nodes and their molecular underpinnings in a given network, potentially suggesting new therapeutic targets and directions. As interoceptive processing drives and influences motivation, emotion, and subsequent behavior, consideration of interoception is important for our understanding of processes that drive ongoing drinking and relapse.

Sex- and subtype-specific adaptations in excitatory signaling onto deep-layer prelimbic cortical pyramidal neurons after chronic alcohol exposure

Long-term alcohol use results in behavioral deficits including impaired working memory, elevated anxiety, and blunted inhibitory control that is associated with prefrontal cortical (PFC) dysfunction. Preclinical observations demonstrate multiple impairments in GABAergic neurotransmission onto deep-layer principal cells (PCs) in the prelimbic cortex that suggest dependence-related cortical dysfunction is the product of elevated excitability in these cells. Despite accumulating evidence showing alcohol-induced changes in interneuron signaling onto PCs differ between sexes, there is limited data explicitly evaluating sex-specific ethanol effects on excitatory signaling onto deep-layer PCs that may further contribute to deficits in PFC-dependent behaviors. To address this, we conducted electrophysiological and behavioral tests in both male and female Sprague-Dawley rats to evaluate the effects of chronic ethanol exposure. Among our observations, we report a marked enhancement in glutamatergic signaling onto deep-layer PCs in male, but not female, rats after alcohol exposure. This phenomenon was furthermore specific to a sub-class of PC, sub-cortically projecting Type-A cells, and coincided with enhanced anxiety-like behavior, but no observable deficit in working memory. In contrast, female rats displayed alcohol-induced facilitation in working memory performance with no change in expression of anxiety-like behavior. Together, these results suggest fundamental differences in alcohol effects on cell activity, cortical sub-circuits, and PFC-dependent behaviors across male and female rats.

The synthetically produced predator odor 2,5-dihydro-2,4,5-trimethylthiazoline increases alcohol self-administration and alters basolateral amygdala response to alcohol in rats

Post-traumatic stress disorder (PTSD) is a psychiatric illness that can increase the risk for developing an alcohol use disorder (AUD). While clinical data has been useful in identifying similarities in the neurobiological bases of these disorders, preclinical models are essential for understanding the mechanism(s) by which stressors increase the risk for escalated alcohol consumption. The purpose of these studies was to examine if exposure of male Long-Evans rats to the synthetically derived predator odor 2,5-dihydro-2,4,5-trimethylthiazoline (TMT; a component of fox feces) would increase sweetened alcohol self-administration, potentially by facilitating transfer of salience towards cues, and alter neuronal response to alcohol as measured by the immediate early gene c-Fos. In experiment 1, rats exposed to repeated (4×) TMT showed reductions in port entries in Pavlovian conditioned approach and increases in sweetened alcohol self-administration. In experiment 2, rats exposed to repeated TMT showed blunted basolateral amygdala c-Fos response to alcohol. In experiment 3, rats exposed to single, but not repeated TMT, showed increases in sweetened alcohol self-administration, and no change in anxiety-like behavior or hyperarousal. In experiment 4, rats continued to show a significant corticosterone response to TMT after repeated exposures. In summary, exposure of male rats to TMT can cause escalations in sweetened alcohol self-administration and reduction in BLA response to alcohol. These studies outline and utilize a novel preclinical model that can be used to further neurobiological understanding of the emergence of escalated alcohol consumption following stress exposure.

Environmental enrichment prevents chronic stress-induced brain-gut axis dysfunction through a GR-mediated mechanism in the central nucleus of the amygdala

BACKGROUND

Cognitive behavioral therapy (CBT) improves quality of life of patients with irritable bowel syndrome (IBS), a disorder characterized by chronic visceral pain and abnormal bowel habits. Whether CBT can actually improve visceral pain in IBS patients is still unknown. The aim of this study is to evaluate whether environment enrichment (EE), the animal analog of CBT, can prevent stress-induced viscero-somatic hypersensitivity through changes in glucocorticoid receptor (GR) signaling within the central nucleus of the amygdala (CeA).

METHODS

Rats were housed in either standard housing (SH) or EE for 7 days before and during daily water avoidance stress (WAS) exposure (1-h/d for 7 days). In the first cohort, visceral and somatic sensitivity were assessed via visceromotor response to colorectal distention and von Frey Anesthesiometer 24 hous and 21 days after WAS. In another cohort, the CeA was isolated for GR mRNA quantification.

KEY RESULTS

Environment enrichment for 7 days before and during the 7 days of WAS persistently attenuated visceral and somatic hypersensitivity when compared to rats placed in SH. Environment enrichment exposure also prevented the WAS-induced decrease in GR expression in the CeA.

CONCLUSION & INFERENCES

Pre-exposure to short-term EE prevents the stress-induced downregulation of GR, and inhibits visceral and somatic hypersensitivity induced by chronic stress. These results suggest that a positive environment can ameliorate stress-induced pathology and provide a non-pharmacological therapeutic option for disorders such as IBS.

Intravenous self-administration of alcohol in rats-problems with translation to humans

Alcohol is consumed orally by humans, and oral self-administration has been successfully modeled in laboratory animals. Over the last several years, attempts have been made to develop a procedure for the reliable intravenous (IV) self-administration of alcohol in rodents. IV self-administration would provide a better tool for investigating neurobiological mechanisms of alcohol reinforcement and dependence because confounding factors associated with oral self-administration, such as variations in orosensory sensitivity to alcohol and/or its absorption, are avoided. A review of the literature shows that rats, mice and non-human primates can initiate and maintain IV self-administration of alcohol. However, there are 50- to 100-fold interspecies differences in the reported alcohol infusion doses required. Most surprising is that the infusion dose (1-2 mg/kg) that reliably maintains IV alcohol self-administration in rats results in total alcohol intakes of only 20-25 mg/kg/hour, which are unlikely to have significant pharmacological effects. The evidence to support IV self-administration of such low doses of alcohol in rats as well as the potential biological mechanisms underlying such self-administration are discussed. The minute amounts of alcohol shown to reliably maintain IV self-administration behavior in rats challenge the relationship between their blood alcohol levels and the rewarding and reinforcing effects of alcohol.

Modulation of sensitivity to alcohol by cortical and thalamic brain regions

The nucleus accumbens core (AcbC) is a key brain region known to regulate the discriminative stimulus/interoceptive effects of alcohol. As such, the goal of the present work was to identify AcbC projection regions that may also modulate sensitivity to alcohol. Accordingly, AcbC afferent projections were identified in behaviorally naïve rats using a retrograde tracer which led to the focus on the medial prefrontal cortex (mPFC), insular cortex (IC) and rhomboid thalamic nucleus (Rh). Next, to examine the possible role of these brain regions in modulating sensitivity to alcohol, neuronal response to alcohol in rats trained to discriminate alcohol (1 g/kg, intragastric [IG]) vs. water was examined using a two-lever drug discrimination task. As such, rats were administered water or alcohol (1 g/kg, IG) and brain tissue was processed for c-Fos immunoreactivity (IR), a marker of neuronal activity. Alcohol decreased c-Fos IR in the mPFC, IC, Rh and AcbC. Lastly, site-specific pharmacological inactivation with muscimol + baclofen (GABA_A agonist + GABA_B agonist) was used to determine the functional role of the mPFC, IC and Rh in modulating the interoceptive effects of alcohol in rats trained to discriminate alcohol (1 g/kg, IG) vs. water. mPFC inactivation resulted in full substitution for the alcohol training dose, and IC and Rh inactivation produced partial alcohol-like effects, demonstrating the importance of these regions, with known projections to the AcbC, in modulating sensitivity to alcohol. Together, these data demonstrate a site of action of alcohol and the recruitment of cortical/thalamic regions in modulating sensitivity to the interoceptive effects of alcohol.

Gabapentin potentiates sensitivity to the interoceptive effects of alcohol and increases alcohol self-administration in rats

Gabapentin, a drug used in the treatment of epileptic seizures and neuropathic pain, has shown efficacy in the treatment of alcohol dependence. Moreover, given that gabapentin is used in the general population (e.g., non-dependent individuals, social drinkers), we sought to utilize preclinical assessments to examine the effects of gabapentin on sensitivity to moderate alcohol doses and alcohol self-administration in rats with a history of moderate drinking. To this end, we assessed whether gabapentin (0, 10, 30, 120 mg/kg, IG) pretreatment alters sensitivity to experimenter- and self-administered alcohol, and whether gabapentin alone has alcohol-like discriminative stimulus effects in rats trained to discriminate alcohol dose (1 g/kg, IG) vs. water. Second, we assessed whether gabapentin (0, 10, 30, 60 mg/kg, IG) would alter alcohol self-administration. Gabapentin pretreatment potentiated the interoceptive effects of both experimenter-administered and self-administered alcohol in discrimination-trained rats. Additionally, the highest gabapentin doses tested (30 and 120 mg/kg) were found to have partial alcohol-like discriminative stimulus effects when administered alone (e.g., without alcohol). In the self-administration trained rats, gabapentin pretreatment (60 mg/kg) resulted in an escalation in alcohol self-administration. Given the importance of interoceptive drug cues in priming and maintaining self-administration, these data define a specific behavioral mechanism (i.e., potentiation of alcohol effects) by which gabapentin may increase alcohol self-administration in non-dependent populations.

Activation of mGluR2/3 following stress hormone exposure restores sensitivity to alcohol in rats

Sensitivity to the interoceptive effects of alcohol is blunted following a period of exposure to the stress hormone corticosterone (CORT), an effect that is suggested to be related, in part, to glutamatergic neuroadaptations. Group II metabotropic glutamate receptors (subtypes 2 and 3; mGluR2/3) modulate several drug- and alcohol-related behaviors, including the interoceptive (discriminative stimulus) effects of alcohol. Therefore, we sought to determine if manipulation of mGluR2/3 would restore sensitivity to the interoceptive effects of alcohol following CORT exposure. Using a two-lever drug discrimination task, male Long-Evans rats were trained to discriminate alcohol (1 g/kg, intragastric [IG]) vs. water. First, the effect of mGluR2/3 antagonism on the discriminative stimulus effects of alcohol was determined using LY341495 (0.3-3.0 mg/kg; intraperitoneal [IP]). Next, the effects of mGluR2/3 antagonism and activation were assessed in discrimination-trained animals exposed to CORT (300 μg/mL) in the home cage drinking water or water only, for 7 days. Following CORT exposure, decreased sensitivity to alcohol (1 g/kg) was observed. Pretreatment with the mGluR2/3 agonist LY379268 (1.0-3.0 mg/kg; IP), but not the mGluR2/3 antagonist (0.3-1.0 mg/kg; IP), restored sensitivity to alcohol. Additionally, in water controls, mGluR2/3 antagonism and mGluR2/3 activation disrupted expression of the discriminative stimulus effects of alcohol. Together, these findings suggest that blunted sensitivity to the interoceptive effects of alcohol following an episode of heightened stress hormone levels may be due to adaptations in mGluR2/3-related systems. The ability of mGluR2/3 activation to restore sensitivity to alcohol under these conditions lends further support for the importance of these receptors under stress-related conditions.

Null mutation of 5α-reductase type I gene alters ethanol consumption patterns in a sex-dependent manner

The neuroactive steroid allopregnanolone (ALLO) is a positive modulator of GABAA receptors, and manipulation of neuroactive steroid levels via injection of ALLO or the 5α-reductase inhibitor finasteride alters ethanol self-administration patterns in male, but not female, mice. The Srd5a1 gene encodes the enzyme 5α-reductase-1, which is required for the synthesis of ALLO. The current studies investigated the influence of Srd5a1 deletion on voluntary ethanol consumption in male and female wildtype (WT) and knockout (KO) mice. Under a continuous access condition, 6 and 10 % ethanol intake was significantly greater in KO versus WT females, but significantly lower in KO versus WT males. In 2-h limited access sessions, Srd5a1 deletion retarded acquisition of 10 % ethanol intake in female mice, but facilitated it in males, versus respective WT mice. The present findings demonstrate that the Srd5a1 gene modulates ethanol consumption in a sex-dependent manner that is also contingent upon ethanol access condition and concentration.

Stress hormone exposure reduces mGluR5 expression in the nucleus accumbens: functional implications for interoceptive sensitivity to alcohol

Escalations in alcohol drinking associated with experiencing stressful life events and chronic life stressors may be related to altered sensitivity to the interoceptive/subjective effects of alcohol. Indeed, through the use of drug discrimination methods, rats show decreased sensitivity to the discriminative stimulus (interoceptive) effects of alcohol following exposure to the stress hormone corticosterone (CORT). This exposure produces heightened elevations in plasma CORT levels (eg, as may be experienced by an individual during stressful episodes). We hypothesized that decreased sensitivity to alcohol may be related, in part, to changes in metabotropic glutamate receptors-subtype 5 (mGluR5) in the nucleus accumbens, as these receptors in this brain region are known to regulate the discriminative stimulus effects of alcohol. In the accumbens, we found reduced mGluR5 expression (immunohistochemistry and Western blot) and decreased neural activation (as measured by c-Fos immunohistochemistry) in response to a moderate alcohol dose (1 g/kg) following CORT exposure (7 days). The functional role of these CORT-induced adaptations in relation to the discriminative stimulus effects of alcohol was confirmed, as both the systemic administration of 3-Cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB) an mGluR5 positive allosteric modulator and the intra-accumbens administration of (R,S)-2-Amino-2-(2-chloro-5-hydroxyphenyl)acetic acid sodium salt (CHPG) an mGluR5 agonist restored sensitivity to alcohol in discrimination-trained rats. These results suggest that activation of mGluR5 may alleviate the functional impact of the CORT-induced downregulation of mGluR5 in relation to sensitivity to alcohol. Understanding the contribution of such neuroadaptations to the interoceptive effects of alcohol may enrich our understanding of potential changes in subjective sensitivity to alcohol during stressful episodes.

Adrenal steroid hormones and ethanol self-administration in male rhesus macaques

RATIONALE

Hypothalamic-pituitary-adrenal (HPA) axis hormones have neuroactive metabolites with receptor activity similar to ethanol.

OBJECTIVES

The present study related HPA hormones in naïve monkeys to ethanol self-administration.

METHODS

Morning plasma adrenocorticotropic hormone (ACTH), cortisol, deoxycorticosterone (DOC), aldosterone, and dehydroepiandrosterone-sulfate (DHEA-S) were measured longitudinally in male rhesus macaques (Macaca mulatta) induced to drink ethanol followed by access to ethanol (4 % w/v, in water) and water 22 h/day for 12 months.

RESULTS

During ethanol access, DOC increased among non-heavy (average intake over 12 months ≤3.0 g/kg/day, n = 23) but not among heavy drinkers (>3.0 g/kg/day, n = 9); aldosterone was greater among heavy drinkers after 6 months. The ratio of DOC/aldosterone decreased only among heavy drinkers after 6 or12 months of ethanol self-administration. ACTH only correlated significantly with DHEA-S, the ratio of cortisol/DHEA-S and DOC after the onset of ethanol access, the former two just in heavy drinkers. Baseline hormones did not predict subsequent ethanol intake over 12 months, but baseline DOC correlated with average blood-ethanol concentrations (BECs), among all monkeys and heavy drinkers as a group. During ethanol access, aldosterone and DOC correlated and tended to correlate, respectively, with 12-month average ethanol intake.

CONCLUSIONS

Ethanol self-administration lowered ACTH and selectively altered its adrenocortical regulation. Mineralocorticoids may compensate for adrenocortical adaptation among heavy drinkers and balance fluid homeostasis. As DOC was uniquely predictive of future BEC and not water intake, to the exclusion of aldosterone, GABAergic neuroactive metabolites of DOC may be risk factors for binge drinking to intoxication.

Age differences in ethanol discrimination: acquisition and ethanol dose generalization curves following multiple training conditions in adolescent and adult rats

BACKGROUND

Adolescents and adults vary in sensitivity to many effects of ethanol (EtOH), although it is unknown whether they also differ in their perception of EtOH's subjective cues. This study characterized EtOH discrimination in adolescent and adult male rats using a rapidly acquired Pavlovian conditioned approach procedure.

METHODS

EtOH at 1 of the 3 training doses (0.75, 1.0, or 1.25 g/kg) served as either a positive (POS) or negative (NOS) occasion setter. Each 20-minute training session consisted of eight 15-second presentations of 2 cue lights located on either side of a dipper delivering chocolate Boost(®) . For POS-trained rats, the cue lights reliably predicted 5-second presentations of chocolate Boost during EtOH but not saline sessions, with the opposite contingencies used for NOS-trained rats. Anticipatory approach behavior (head entries into the reward delivery area) in the presence and absence of the cue lights was used to calculate discrimination scores on EtOH and saline sessions. Following acquisition, various doses of EtOH (0 to 1.5 g/kg) were administered to establish generalization curves.

RESULTS

Although animals of both ages responded differentially on EtOH and saline sessions by the end of acquisition, adults met criterion more quickly and had higher discrimination scores during reinforced sessions than adolescents. Whereas adolescents failed to demonstrate any dose-dependent responding during testing when trained with the 0.75 or 1.25 g/kg EtOH doses, adults demonstrated broader EtOH generalization during testing sessions following training with all 3 EtOH doses. Among adolescents trained with 1.0 g/kg EtOH, less generalization occurred relative to adults.

CONCLUSIONS

Adolescents were less sensitive to EtOH's interoceptive effects, indicating that EtOH is likely a more salient cue for adults than for adolescents. These findings contribute to evidence that suggests adolescent-typical insensitivity to internal cues that typically limit EtOH consumption may contribute to the elevated intake commonly reported during this developmental period.

Transient increase in alcohol self-administration following a period of chronic exposure to corticosterone

Stressful life events and chronic stressors have been associated with escalations in alcohol drinking. Stress exposure leads to the secretion of glucocorticoids (cortisol in the human; corticosterone (CORT) in the rodent). To model a period of heightened elevations in CORT, the present work assessed the effects of chronic exposure to the stress hormone CORT on alcohol self-administration. Male Long Evans rats were trained to self-administer a sweetened alcohol solution (2% sucrose/15% alcohol) resulting in moderate levels of daily alcohol intake (0.5-0.7 g/kg). Following stable baseline operant self-administration, rats received CORT in the drinking water for 7 days. A transient increase in alcohol self-administration was observed on the first self-administration session following CORT exposure, and behavior returned to control levels by the second session. Control experiments determined that this increase in alcohol self-administration was specific to alcohol, unrelated to general motor activation, and functionally dissociated from decreased CORT levels at the time of testing. These results indicate that repeated exposure to heightened levels of stress hormone (e.g., as may be experienced during stressful episodes) has the potential to lead to exacerbated alcohol intake in low to moderate drinkers. Given that maladaptive drinking patterns, such as escalated alcohol drinking following stressful episodes, have the potential to put an individual at risk for future drinking disorders, utilization of this model will be important for examination of neuroadaptations that occur as a consequence of CORT exposure in order to better understand escalated drinking following stressful episodes in nondependent individuals.

Enhanced AMPA receptor activity increases operant alcohol self-administration and cue-induced reinstatement

Long-term alcohol exposure produces neuroadaptations that contribute to the progression of alcohol abuse disorders. Chronic alcohol consumption results in strengthened excitatory neurotransmission and increased α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors (AMPA) receptor signaling in animal models. However, the mechanistic role of enhanced AMPA receptor activity in alcohol-reinforcement and alcohol-seeking behavior remains unclear. This study examined the role of enhanced AMPA receptor function using the selective positive allosteric modulator, aniracetam, in modulating operant alcohol self-administration and cue-induced reinstatement. Male alcohol-preferring (P-) rats, trained to self-administer alcohol (15%, v/v) versus water were pre-treated with aniracetam to assess effects on maintenance of alcohol self-administration. To determine reinforcer specificity, P-rats were trained to self-administer sucrose (0.8%, w/v) versus water, and effects of aniracetam were tested. The role of aniracetam in modulating relapse of alcohol-seeking was assessed using a response contingent cue-induced reinstatement procedure in P-rats trained to self-administer 15% alcohol. Aniracetam pre-treatment significantly increased alcohol-reinforced responses relative to vehicle treatment. This increase was not attributed to aniracetam-induced hyperactivity as aniracetam pre-treatment did not alter locomotor activity. AMPA receptor involvement was confirmed because 6,7-dinitroquinoxaline-2,3-dione (AMPA receptor antagonist) blocked the aniracetam-induced increase in alcohol self-administration. Aniracetam did not alter sucrose-reinforced responses in sucrose-trained P-rats, suggesting that enhanced AMPA receptor activity is selective in modulating the reinforcing function of alcohol. Finally, aniracetam pre-treatment potentiated cue-induced reinstatement of alcohol-seeking behavior versus vehicle-treated P-rats. These data suggest that enhanced glutamate activity at AMPA receptors may be key in facilitating alcohol consumption and seeking behavior, which could ultimately contribute to the development of alcohol abuse disorders.

Assessment of the interoceptive effects of alcohol in rats using short-term training procedures

In the present study, we sought to determine whether the interoceptive effects of alcohol (1 g/kg, IG) could be assessed using a Pavlovian discrimination method, in which the alcohol drug state sets the occasion for which an environmental stimulus (e.g., light) will be followed by a sucrose reward. This procedure takes advantage of a naturally occurring behavior (i.e., food-seeking) which can be trained rapidly prior to the initiation of discrimination training. Given that the interoceptive effects of alcohol are routinely assessed using operant drug discrimination methods, another group of rats was trained using standard two-lever operant drug discrimination procedures in an effort to compare the Pavlovian procedure to a known behavioral benchmark. The results from this work show that, in addition to operant discrimination procedures, a Pavlovian discrimination task can be used to evaluate the interoceptive effects of alcohol. In addition to the brief behavioral sucrose access training (3 days) required prior to the initiation of the Pavlovian discrimination, the alcohol discrimination was acquired relatively rapidly (i.e., 8 training sessions), shortening the overall duration of the experiment. These features of the Pavlovian procedure make it a valuable method by which to assess the interoceptive effects of alcohol if a short experimental time frame is required, such as assessing the interoceptive effects of alcohol during a brief developmental window (e.g., adolescence) or determining the effects of a pretreatment (i.e., chronic stress, chronic drug pretreatment) on the acquisition of the alcohol discrimination. As such, this initial characterization confirms the feasibility of using this Pavlovian discrimination training method as an additional tool by which to assess the interoceptive effects of alcohol, as there may be experimental situations that necessitate short term discrimination training.

Do initial responses to drugs predict future use or abuse?

Individuals vary in their initial reactions to drugs of abuse in ways that may contribute to the likelihood of subsequent drug use. In humans, most drugs of abuse produce positive subjective states such as euphoria and feelings of well-being, which may facilitate repeated use. In nonhumans, many drugs initially increase locomotor activity and produce discriminative stimulus effects, both of which have been considered to be models of human stimulant and subjective states. Both humans and nonhumans vary in their sensitivity to early acute drug effects in ways that may predict future use or self-administration, and some of these variations appear to be genetic in origin. However, it is not known exactly how the initial responses to drugs in either humans or nonhumans relate to subsequent use or abuse. In humans, positive effects of drugs facilitate continued use of a drug while negative effects discourage use, and in nonhumans, greater genetic risk for drug intake is predicted by reduced sensitivity to drug aversive effects; but whether these initial responses affect escalation of drug use, and the development of dependence is currently unknown. Although early use of a drug is a necessary step in the progression to abuse and dependence, other variables may be of greater importance in the transition from use to abuse. Alternatively, the same variables that predict initial acute drug effects and early use may significantly contribute to continued use, escalation and dependence. Here we review the existing evidence for relations between initial direct drug effects, early use, and continued use. Ultimately, these relations can only be determined from systematic longitudinal studies with comprehensive assessments from early drug responses to progression of problem drug use. In parallel, additional investigation of initial responses in animal models as predictors of drug use will shed light on the underlying mechanisms.

Amphetamine modifies ethanol intake of psychosocially stressed male rats

Studies of socially housed rodents have provided significant information regarding the consequences of exposure to stressors. Psychosocial stressors are known to alter the ingestion of ethanol and the activity of the dopaminergic neuronal system. Since both stressors and ethanol are known to affect the function of dopaminergic neurons, we employed amphetamine to assess the role of this neural system on the ingestion of ethanol by psychosocially stressed male rats. Male rats housed two per cage were designated as dominant or subdominant rats based on evaluations of agonistic behavior and body weight changes. The dyad-housed rats and a group of single-housed rats were sequentially assessed for ethanol intake after injections of saline or amphetamine (0.3, 0.9 or 2.7 mg/kg i.p.) both prior to dyad housing and subsequently again during dyad-housing. Prior to dyad housing ethanol intake of future subdominant rats was higher than that of future dominant rats. Dyad-housing significantly increased ethanol intake of dominant rats. Pre-dyad the highest dose of amphetamine potently depressed ethanol ingestion. Sensitivity to amphetamine's depressant effect on ethanol intake was higher at the dyad test in all subjects, most prominently in single-housed rats. In contrast to the single-housed rats, the dyad-housed rats displayed saccharin anhedonia. It can be concluded that dopaminergic system modulates, at least partially, the psychosocial stress-induced changes in ethanol intake. Furthermore, the level of ethanol ingestion at the pre-dyad test was predictive of future hierarchical status.

Effects of alcohol dependence and withdrawal on stress responsiveness and alcohol consumption

A complex relationship exists between alcohol-drinking behavior and stress. Alcohol has anxiety-reducing properties and can relieve stress, while at the same time acting as a stressor and activating the body's stress response systems. In particular, chronic alcohol exposure and withdrawal can profoundly disturb the function of the body's neuroendocrine stress response system, the hypothalamic-pituitary-adrenocortical (HPA) axis. A hormone, corticotropin-releasing factor (CRF), which is produced and released from the hypothalamus and activates the pituitary in response to stress, plays a central role in the relationship between stress and alcohol dependence and withdrawal. Chronic alcohol exposure and withdrawal lead to changes in CRF activity both within the HPA axis and in extrahypothalamic brain sites. This may mediate the emergence of certain withdrawal symptoms, which in turn influence the susceptibility to relapse. Alcohol-related dysregulation of the HPA axis and altered CRF activity within brain stress-reward circuitry also may play a role in the escalation of alcohol consumption in alcohol-dependent individuals. Numerous mechanisms have been suggested to contribute to the relationship between alcohol dependence, stress, and drinking behavior. These include the stress hormones released by the adrenal glands in response to HPA axis activation (i.e., corticosteroids), neuromodulators known as neuroactive steroids, CRF, the neurotransmitter norepinephrine, and other stress-related molecules.