Extracellular dopamine levels are lower in the medial prefrontal cortex of alcohol-preferring rats compared to Wistar rats

SERT and OCT mediate 5-HT1B receptor regulation of immobility behavior and uptake of 5-HT and HIS

5-HT clearance, commonly mediated by transporters in the uptake-1 and uptake-2 families, has been linked to 5-HT1B receptor's action on behaviors. Since no specific transporters identified yet, effects of serotonin transporter (SERT) and organic cation transporter (OCTs) on 5-HT1B-elicited immobility phenotype, and 5-HT and HIS uptake were then investigated. Intraperitoneal injections of SERT inhibitor fluoxetine (FLX) and/or OCTs inhibitor decynium (D22) were used prior to local perfusion of 5-HT1B agonist CP93129 into the ventral hippocampus to measure immobility times in the FST and TST, to measure 5-HT uptake efficiencies and HIS uptake efficiencies derived from linear regressions using the transient no-net-flux quantitative microdialysis in C57BL/6 mice. Exogenous 5-HT and HIS uptake were measured following incubation of FLX and/or D22 with CP93129 in the RBL-2H3 cells. Moreover, surface membrane levels of SERT and OCT were detected in response to CP93129. Local CP93129 prolonged immobility times, which were attenuated following pretreatment of either inhibitor. Local CP93129 lowered the slopes obtained from the lineal regressions for 5-HT and HIS (slope is reciprocal to uptake efficiency), which were then weakened following pretreatment of either inhibitor. Similar findings were obtained following CP93129 incubation, and co-incubation of CP93129 with either inhibitor in the RBL-2H3. Moreover, CP93129 dose-dependently moved SERT and OCT3 in the cytosol to the surface membrane. Both SERT and OCT are the target effectors mediating 5-HT1B regulation of immobility time and 5-HT uptake, OCT mediates 5-HT1B regulation of HIS uptake. Their underlying signal transductions need to be further explored.

Mesolimbic Neural Response Dynamics Predict Future Individual Alcohol Drinking in Mice

BACKGROUND

Individual variability in response to rewarding stimuli is a striking but understudied phenomenon. The mesolimbic dopamine system is critical in encoding the reinforcing properties of both natural reward and alcohol; however, how innate or baseline differences in the response dynamics of this circuit define individual behavior and shape future vulnerability to alcohol remain unknown.

METHODS

Using naturalistic behavioral assays, a voluntary alcohol drinking paradigm, in vivo fiber photometry, in vivo electrophysiology, and chemogenetics, we investigated how differences in mesolimbic neural circuit activity contribute to the individual variability seen in reward processing and, by proxy, alcohol drinking.

RESULTS

We first characterized heterogeneous behavioral and neural responses to natural reward and defined how these baseline responses predicted future individual alcohol-drinking phenotypes in male mice. We then determined spontaneous ventral tegmental area dopamine neuron firing profiles associated with responses to natural reward that predicted alcohol drinking. Using a dual chemogenetic approach, we mimicked specific mesolimbic dopamine neuron firing activity before or during voluntary alcohol drinking to link unique neurophysiological profiles to individual phenotype. We show that hyperdopaminergic individuals exhibit a lower neuronal response to both natural reward and alcohol that predicts lower levels of alcohol consumption in the future.

CONCLUSIONS

These findings reveal unique, circuit-specific neural signatures that predict future individual vulnerability or resistance to alcohol and expand the current knowledge base on how some individuals are able to titrate their alcohol consumption whereas others go on to engage in unhealthy alcohol-drinking behaviors.

Proactive Versus Reactive Control Strategies Differentially Mediate Alcohol Drinking in Male Wistars and P Rats

Problematic alcohol consumption is associated with deficits in decision-making and alterations in prefrontal cortex neural activity likely contribute. We hypothesized that the differences in cognitive control would be evident between male Wistars and a model of genetic risk: alcohol-preferring P rats. Cognitive control is split into proactive and reactive components. Proactive control maintains goal-directed behavior independent of a stimulus, whereas reactive control elicits goal-directed behavior at the time of a stimulus. We hypothesized that Wistars would show proactive control over alcohol seeking whereas P rats would show reactive control over alcohol seeking. Neural activity was recorded from the prefrontal cortex during an alcohol seeking task with two session types. On congruent sessions, the conditioned stimulus (CS+) was on the same side as alcohol access. Incongruent sessions presented alcohol opposite the CS+. Wistars, but not P rats, made more incorrect approaches during incongruent sessions, suggesting that Wistars utilized the previously learned rule. This motivated the hypothesis that neural activity reflecting proactive control would be observable in Wistars but not P rats. While P rats showed differences in neural activity at times of alcohol access, Wistars showed differences prior to approaching the sipper. These results support our hypothesis that Wistars are more likely to engage in proactive cognitive control strategies whereas P rats are more likely to engage in reactive cognitive control strategies. Although P rats were bred to prefer alcohol, the differences in cognitive control may reflect a sequela of behaviors that mirror those in humans at risk for an AUD.

The influence of dopamine autoreceptors on temperament and addiction risk

As a major regulator of dopamine (DA), DA autoreceptors (DAARs) exert substantial influence over DA-mediated behaviors. This paper reviews the physiological and behavioral impact of DAARs. Individual differences in DAAR functioning influences temperamental traits such as novelty responsivity and impulsivity, both of which are associated with vulnerability to addictive behavior in animal models and a broad array of externalizing behaviors in humans. DAARs additionally impact the response to psychostimulants and other drugs of abuse. Human PET studies of D2-like receptors in the midbrain provide evidence for parallels to the animal literature. These data lead to the proposal that weak DAAR regulation is a risk factor for addiction and externalizing problems. The review highlights the potential to build translational models of the functional role of DAARs in behavior. It also draws attention to key limitations in the current literature that would need to be addressed to further advance a weak DAAR regulation model of addiction and externalizing risk.

Flexible coding schemes in dorsomedial prefrontal cortex underlie decision-making during delay discounting

Determining how an agent decides between a small, immediate versus a larger, delayed reward has provided insight into the psychological and neural basis of decision-making. The tendency to excessively discount the value of delayed rewards is thought to reflect deficits in brain regions critical for impulse control such as the prefrontal cortex (PFC). This study tested the hypothesis that dorsomedial PFC (dmPFC) is critically involved in flexibly managing neural representations of strategies that limit impulsive choices. Optogenetic silencing of neurons in the rat dmPFC increased impulsive choices at an 8 sec, but not 4 sec, delay. Neural recordings from dmPFC ensembles revealed that, at the 8-sec delay, the encoding landscape transitions to reflect a deliberative-like process rather than the schema-like processes observed at the 4-sec delay. These findings show that changes in the encoding landscape reflect changes in task demands and that dmPFC is uniquely involved in decisions requiring deliberation.

Proactive Versus Reactive Control Strategies Differentially Mediate Alcohol Drinking in Wistar and P rats

Problematic alcohol consumption is associated with deficits in decision-making, and alterations in prefrontal cortex neural activity likely contributes. We hypothesized that differences in cognitive control would be evident between male Wistar rats and a model for genetic risk for alcohol use disorder (alcohol-preferring P rats). Cognitive control can be split into proactive and reactive components. Proactive control maintains goal-directed behavior independent of a stimulus whereas reactive control elicits goal-directed behavior at the time of a stimulus. We hypothesized that Wistars would show proactive control over alcohol-seeking whereas P rats would show reactive control over alcohol-seeking. Neural ensembles were recorded from prefrontal cortex during an alcohol seeking task that utilized two session types. On congruent sessions the CS+ was on the same side as alcohol access. Incongruent sessions presented alcohol opposite the CS+. Wistars, but not P rats, exhibited an increase in incorrect approaches during incongruent sessions, suggesting that Wistars utilized the previously learned task-rule. This motivated the hypothesis that ensemble activity reflecting proactive control would be observable in Wistars but not P rats. While P rats showed differences in neural activity at times relevant for alcohol delivery, Wistars showed differences prior to approaching the sipper. These results support our hypothesis that Wistars are more likely to engage proactive cognitive-control strategies whereas P rats are more likely to engage reactive cognitive control strategies. Although P rats were bred to prefer alcohol, differences in cognitive control may reflect a sequela of behaviors that mirror those in humans at risk for an AUD.

Differential effects of quinine adulteration of alcohol on seeking and drinking

Alcohol dependence is characterized by compulsive alcohol use. Alcohol-paired stimuli can drive compulsive alcohol use, induce craving, and lead to relapse. Alcohol dependence is highly heritable, and individuals with a family history are at elevated risk to develop an alcohol use disorder. Understanding the association between genetic vulnerability to alcohol dependence and neural alterations that promote an addiction phenotype are critical to the prevention and treatment of alcohol dependence. Here we use selectively bred alcohol-preferring P rats and their progenitor strain, Wistar rats, to investigate the relationship between genetic liability and alcohol-seeking and drinking behaviors in a discriminative stimuli paradigm. To further investigate strain differences in motivated responding, alcohol was adulterated with quinine, and intake and responding were assessed. While both strains learned to discriminate between stimuli that predicted alcohol availability, P rats learned faster and consumed more alcohol. Quinine adulteration reduced ethanol intake in both strains with no effect on ethanol-seeking measures. These data suggest genetic vulnerability to alcohol dependence is associated with increased motivated behaviors and highlight the utility of P rats in teasing apart the neural mechanisms associated with this phenotype. Additionally, these data suggest a dissociation between the neural systems that engage ethanol drinking versus compulsive ethanol seeking.

The reinforcing effects of ethanol within the prelimbic cortex and ethanol drinking: Involvement of local dopamine D2 receptor-mediated neurotransmission

Previous studies have identified important mesolimbic regions in supporting the reinforcing effects of ethanol. However, the involvement of the medial prefrontal cortex (mPFC), another key region within the mesocorticolimbic system, in ethanol reinforcement has been understudied. The objective of the current study was to examine the role of the prelimbic (PL) cortex sub-region of the mPFC in ethanol reinforcement and drinking. Intracranial self-administration was used to examine the reinforcing effects of ethanol within the PL cortex. Quantitative microdialysis was used to measure basal extracellular DA concentrations and clearance in the PL cortex following chronic ethanol drinking. In addition, the involvement of dopamine (DA) D2 receptors within the PL cortex on the reinforcing effects of ethanol and ethanol drinking was determined. Ethanol was dose-dependent self-administered into the PL cortex, with significantly more infusions elicited by 100-200 mg% ethanol than vehicle. Co-infusion of the D2 receptor antagonist sulpiride significantly reduced ethanol self-administration. Chronic ethanol drinking significantly elevated basal extracellular DA concentrations without altering DA clearance. Microinjection of sulpiride into the PL cortex selectively reduced ethanol, but not saccharine, drinking. These results indicate that the PL cortex supported the reinforcing effects of ethanol, and that ethanol drinking enhanced basal DA neurotransmission within the PL cortex. In addition, D2 receptor antagonism within the PL cortex reduced ethanol self-administration and drinking. Collectively, these findings revealed important DA mechanisms within the PL cortex in mediating ethanol reinforcement and drinking.

Alcohol-preferring P rats exhibit aversion-resistant drinking of alcohol adulterated with quinine

Understanding why some people continue to drink alcohol despite negative consequences and others do not is a central problem in the study of alcohol use disorder (AUD). In this study, we used alcohol-preferring P rats (a strain bred to prefer to drink alcohol, a model for genetic risk for AUD) and Wistar rats (control) to examine drinking despite negative consequences in the form of an aversive bitter taste stimulus produced by quinine. Animals were trained to consume 10% ethanol in a simple Pavlovian conditioning task that paired alcohol access with an auditory stimulus. When the alcohol was adulterated with quinine (0.1 g/L), P rats continued to consume alcohol + quinine at the same rate as unadulterated alcohol, despite a demonstrated aversion to quinine-adulterated alcohol when given a choice between adulterated and unadulterated alcohol in the home cage. Conversely, Wistar rats decreased consumption of quinine-adulterated alcohol in the task, but continued to try the alcohol + quinine solution at similar rates to unadulterated alcohol. These results indicate that following about 8 weeks of alcohol consumption, P rats exhibit aversion-resistant drinking. This model could be used in future work to explore how the biological basis of alcohol consumption and genetic risk for excessive drinking lead to drinking that is resistant to devaluation.

Co-administration of ethanol and nicotine heightens sensitivity to ethanol reward within the nucleus accumbens (NAc) shell and increasing NAc shell BDNF is sufficient to enhance ethanol reward in naïve Wistar rats

Alcohol use disorder most commonly presents as a polydrug disorder where greater than 85% are estimated to smoke. EtOH and nicotine (NIC) co-abuse or exposure results in unique neuroadaptations that are linked to behaviors that promote drug use. The current experiments aimed to identify neuroadaptations within the mesolimbic pathway produced by concurrent EtOH and NIC exposure. The experiments used four overall groups of male Wistar rats consisting of vehicle, EtOH or NIC alone, and EtOH+NIC. Drug exposure through direct infusion into the posterior ventral tegmental area (pVTA) stimulated release of glutamate and dopamine in the nucleus accumbens (NAc) shell, which was quantified through high-performance liquid chromatography. Additionally, brain-derived neurotrophic factor (BDNF) protein levels were measured via enzyme-linked immunosorbent assay (ELISA). A second experiment investigated the effects of drug pretreatment within the pVTA on the reinforcing properties of EtOH within the NAc shell through intracranial self-administration (ICSA). The concluding experiment evaluated the effect of NAc shell pretreatment with BDNF on EtOH reward utilizing ICSA within that region. The data indicated that only EtOH+NIC administration into the pVTA simultaneously increased glutamate, dopamine, and BDNF in the NAc shell. Moreover, only pVTA pretreatment with EtOH+NIC enhanced the reinforcing properties of EtOH in the NAc shell. BDNF pretreatment in the NAc shell was also sufficient to enhance the reinforcing properties of EtOH in the NAc shell. The collected data suggest that concurrent EtOH+NIC exposure results in a distinct neurochemical response and neuroadaptations within the mesolimbic pathway that alter EtOH reward.

Evidence for incentive salience sensitization as a pathway to alcohol use disorder

The incentive salience sensitization (ISS) theory of addiction holds that addictive behavior stems from the ability of drugs to progressively sensitize the brain circuitry that mediates attribution of incentive salience (IS) to reward-predictive cues and its behavioral manifestations. In this article, we establish the plausibility of ISS as an etiological pathway to alcohol use disorder (AUD). We provide a comprehensive and critical review of evidence for: (1) the ability of alcohol to sensitize the brain circuitry of IS attribution and expression; and (2) attribution of IS to alcohol-predictive cues and its sensitization in humans and non-human animals. We point out gaps in the literature and how these might be addressed. We also highlight how individuals with different alcohol subjective response phenotypes may differ in susceptibility to ISS as a pathway to AUD. Finally, we discuss important implications of this neuropsychological mechanism in AUD for psychological and pharmacological interventions attempting to attenuate alcohol craving and cue reactivity.

COMT Inhibition Alters Cue-Evoked Oscillatory Dynamics during Alcohol Drinking in the Rat

Alterations in the corticostriatal system have been implicated in numerous substance use disorders, including alcohol use disorder (AUD). Adaptations in this neural system are associated with enhanced drug-seeking behaviors following exposure to cues predicting drug availability. Therefore, understanding how potential treatments alter neural activity in this system could lead to more refined and effective approaches for AUD. Local field potentials (LFPs) were acquired simultaneously in the prefrontal cortex (PFC) and nucleus accumbens (NA) of both alcohol preferring (P) and Wistar rats engaged in a Pavlovian conditioning paradigm wherein a light cue signaled the availability of ethanol (EtOH). On test days, the catechol-o-methyl-transferase (COMT) inhibitor tolcapone was administered prior to conditioning. Stimulus-evoked voltage changes were observed following the presentation of the EtOH cue in both strains and were most pronounced in the PFC of P rats. Phase analyses of LFPs in the θ band (5–11 Hz) revealed that PFC-NA synchrony was reduced in P rats relative to Wistars but was robustly increased during drinking. Presentation of the cue resulted in a larger phase reset in the PFC of P rats but not Wistars, an effect that was attenuated by tolcapone. Additionally, tolcapone reduced cued EtOH intake in P rat but not Wistars. These results suggest a link between corticostriatal synchrony and genetic risk for excessive drinking. Moreover, inhibition of COMT within these systems may result in reduced attribution of salience to reward paired stimuli via modulation of stimulus-evoked changes to cortical oscillations in genetically susceptible populations.

Selective breeding for high alcohol consumption and response to nicotine: locomotor activity, dopaminergic in the mesolimbic system, and innate genetic differences in male and female alcohol-preferring, non-preferring, and replicate lines of high-alcohol drinking and low-alcohol drinking rats

RATIONALE

There is evidence for a common genetic link between alcohol and nicotine dependence. Rodents selectively bred for high alcohol consumption/responsivity are also more likely to self-administer nicotine than controls.

OBJECTIVES

The experiments examined the response to systemic nicotine, the effects of nicotine within the drug reward pathway, and innate expression of nicotine-related genes in a brain region regulating drug reward/self-administration in multiple lines of rats selectively bred for high and low alcohol consumption.

METHODS

The experiments examined the effects of systemic administration of nicotine on locomotor activity, the effects of nicotine administered directly into the (posterior ventral tegmental area; pVTA) on dopamine (DA) release in the nucleus accumbens shell (AcbSh), and innate mRNA levels of acetylcholine receptor genes in the pVTA were determined in 6 selectively bred high/low alcohol consuming and Wistar rat lines.

RESULTS

The high alcohol-consuming rat lines had greater nicotine-induced locomotor activity compared to low alcohol-consuming rat lines. Microinjections of nicotine into the pVTA resulted in DA release in the AcbSh with the dose response curves for high alcohol-consuming rats shifted leftward and upward. Genetic analysis of the pVTA indicated P rats expressed higher levels of α2 and β4.

CONCLUSION

Selective breeding for high alcohol preference resulted in a genetically divergent behavioral and neurobiological sensitivity to nicotine. The observed behavioral and neurochemical differences between the rat lines would predict an increased likelihood of nicotine reinforcement. The data support the hypothesis of a common genetic basis for drug addiction and identifies potential receptor targets.

Differential COMT expression and behavioral effects of COMT inhibition in male and female Wistar and alcohol preferring rats

Polymorphisms of the catechol-O-methyl transferase (COMT) gene have been associated with alcoholism, suggesting that alterations in the metabolism of catecholamines may be a critical component of the neuropathology of alcoholism. In the current experiments, the COMT inhibitor tolcapone was utilized in an operant behavioral model of reinforcer-seeking and drinking to determine if this compound was capable of remediating the excessive seeking and drinking phenotype of the alcohol-preferring P rat. Tolcapone was administered to male and female alcohol-reinforced P and Wistar rats. Additionally, tolcapone was administered to male sucrose-reinforced P and Wistar rats to determine if its effects also extended to a natural reinforcer. Animals were trained to make an operant response that resulted in 20 min uninterrupted access to the reinforcer solutions. Tolcapone had no effect in female rats on either seeking or consumption of ethanol. However, reductions of both reinforcer seeking and consumption were observed in male P rats, but only of seeking in Wistars. In separate experiments, using reinforcer naïve male and female animals, COMT expression was assessed via Western Blot analysis. Sex differences in COMT expression were also observed, where male P rats exhibited a marked reduction in protein expression relative to females in the PFC. Sex differences were not observed for Wistars or in the striatum and hippocampus. These data complement our previous findings in which tolcapone reduced cue-evoked responses in P rats and further suggest clinical utility of COMT inhibitors in the treatment of addiction disorders, specifically in male high drinkers.

Impulsivity in rodents with a genetic predisposition for excessive alcohol consumption is associated with a lack of a prospective strategy

Increasing evidence supports the hypothesis that impulsive decision-making is a heritable risk factor for an alcohol use disorder (AUD). Clearly identifying a link between impulsivity and AUD risk, however, is complicated by the fact that both AUDs and impulsivity are heterogeneous constructs. Understanding the link between the two requires identifying the underlying cognitive factors that lead to impulsive choices. Rodent models have established that a family history of excessive drinking can lead to the expression of a transgenerational impulsive phenotype, suggesting heritable alterations in the decision-making process. In the present study, we explored the cognitive processes underlying impulsive choice in a validated, selectively bred rodent model of excessive drinking-the alcohol-preferring ("P") rat. Impulsivity was measured via delay discounting (DD), and P rats exhibited an impulsive phenotype as compared to their outbred foundation strain-Wistar rats. Steeper discounting in P rats was associated with a lack of a prospective behavioral strategy, which was observed in Wistar rats and was directly related to DD. To further explore the underlying cognitive factors mediating these observations, a drift diffusion model of DD was constructed. These simulations supported the hypothesis that prospective memory of the delayed reward guided choice decisions, slowed discounting, and optimized the fit of the model to the experimental data. Collectively, these data suggest that a deficit in forming or maintaining a prospective behavioral plan is a critical intermediary to delaying reward, and by extension, may underlie the inability to delay reward in those with increased AUD risk.

Adolescent Alcohol Exposure Persistently Impacts Adult Neurobiology and Behavior

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

Medial Prefrontal Cortical Dopamine Responses During Operant Self-Administration of Sweetened Ethanol

BACKGROUND

Medial prefrontal cortex (mPFC) dysfunction is present in heavy alcohol consumers. Dopamine signaling in mPFC is associated with executive functioning and affects drinking behavior; however, direct measurement of extracellular mPFC dopamine during appetitive and consummatory ethanol (EtOH) self-administration behavior has not been reported.

METHODS

We used in vivo microdialysis in freely behaving, adult, male, Long Evans rats to determine extracellular dopamine concentration in the mPFC during operant self-administration of an EtOH-plus-sucrose or sucrose solution. The model separated appetitive/seeking from consummatory phases of the operant session. Dopamine was also monitored in an untrained handling control group, and dialysate EtOH was measured in the EtOH-drinking group.

RESULTS

Home cage baseline dopamine was lower in rats that experienced a week of drinking sweetened EtOH compared with sucrose-drinking and handling controls. Transfer into the operant chamber and the initiation of consumption stimulated a relatively higher change in dopamine over baseline in the sweetened EtOH group compared with sucrose and handling controls. However, all groups show a dopamine response during transfer into the operant chamber, and the sucrose group had a relatively higher change in dopamine over baseline during initiation of consumption compared with handling controls. The time courses of dopamine and EtOH in the mPFC differ in the EtOH-consuming rats.

CONCLUSIONS

Differences in extracellular mPFC dopamine between EtOH drinkers compared with control groups suggest that mPFC dopamine is involved in the mechanism of operant self-administration of sweetened EtOH and sucrose. Furthermore, the increase in dopamine during consumption is consistent with a role of mPFC dopamine in reward prediction.

A Genetic Animal Model of Alcoholism for Screening Medications to Treat Addiction

The purpose of this review is to present up-to-date pharmacological, genetic, and behavioral findings from the alcohol-preferring P rat and summarize similar past work. Behaviorally, the focus will be on how the P rat meets criteria put forth for a valid animal model of alcoholism with a highlight on its use as an animal model of polysubstance abuse, including alcohol, nicotine, and psychostimulants. Pharmacologically and genetically, the focus will be on the neurotransmitter and neuropeptide systems that have received the most attention: cholinergic, dopaminergic, GABAergic, glutamatergic, serotonergic, noradrenergic, corticotrophin releasing hormone, opioid, and neuropeptide Y. Herein, we sought to place the P rat's behavioral and neurochemical phenotypes, and to some extent its genotype, in the context of the clinical literature. After reviewing the findings thus far, this chapter discusses future directions for expanding the use of this genetic animal model of alcoholism to identify molecular targets for treating drug addiction in general.

Desipramine enhances the ability of paliperidone to decrease alcohol drinking

Alcohol use disorder commonly occurs in patients with schizophrenia and dramatically worsens their course. The atypical antipsychotic clozapine has been associated with reduced drinking in these patients, but its toxicity reduces its use. We have attempted to create a clozapine-like drug by combining agents that capture components of clozapine's pharmacologic action, including its weak dopamine D2 blockade and noradrenergic modulation. The current study assessed whether paliperidone, a dopamine D2 receptor and adrenergic alpha-2 receptor antagonist like clozapine, would attenuate alcohol drinking in the alcohol-preferring P rat and the Syrian golden hamster, and whether desipramine, a norepinephrine reuptake inhibitor, would potentiate the ability of paliperidone to attenuate alcohol drinking in the P rat and the Syrian golden hamster. Daily subcutaneous injections of paliperidone (5 mg/kg for the rat; 1 mg/kg for the hamster) over 20 days slightly and transiently attenuated initiation of alcohol consumption in both animals. Desipramine (3 mg/kg) or lower doses of paliperidone alone did not affect alcohol drinking. However, the combination of desipramine (3 mg/kg) and paliperidone essentially prevented initiation of alcohol drinking and acquisition of alcohol preference in the P rat (2.5 or 5 mg/kg), and almost as dramatically suppressed chronic alcohol intake and alcohol preference in the hamster (2.5 mg/kg). Taken together, the current data suggest that (1) the desipramine and paliperidone combination attenuates alcohol drinking in a synergistic manner, and (2) desipramine and paliperidone may serve as an effective new treatment for alcohol use disorder in patients with schizophrenia.

Neural Firing in the Prefrontal Cortex During Alcohol Intake in Alcohol-Preferring "P" Versus Wistar Rats

BACKGROUND

Neural activity within the prefrontal cortex (PFC) is altered by alcohol and alcohol-associated stimuli and is mediated by genetic susceptibility to alcoholism. However, very little is known about how genetic risk of excessive drinking might mediate neural firing in the PFC during alcohol consumption.

METHODS

To determine how genetic risk influences alcohol seeking, intake, and neural activity, a Pavlovian alcohol consumption task was used-the 2-Way Cued Access Protocol (2CAP). Alcohol-preferring "P" rats and relatives of their (heterogeneous) founding Wistar population were used for these studies. After acquisition of 2CAP, extinction of responding for alcohol was evaluated by substituting water for alcohol. Following these experiments, in vivo electrophysiological recordings were obtained during 2CAP from the PFC in a separate cohort of Wistar and P rats implanted with moveable tetrode microdrives.

RESULTS

P and Wistar rats increased daily alcohol seeking and intake with P rats consuming roughly twice as much alcohol as Wistar. Both rat populations decreased seeking behavior during extinction. However, P rats displayed persistent increases in seeking after controlling for intake versus Wistar. Higher firing rates (FRs) were observed in P rats prior to 2CAP and throughout alcohol and water consumption compared with Wistars that were matched for alcohol-drinking history. Differences in FR were driven, in part, by a larger percentage of neurons in P rats versus Wistars that increased FR compared with those that decreased, or did not change.

CONCLUSIONS

These data provide additional evidence of increased alcohol consumption and persistent alcohol seeking in P versus Wistar rats. Differences in PFC neural firing observed in P rats prior to drinking could be heritable and/or related to an enhanced response to alcohol-associated contextual cues. FR differences observed during alcohol drinking might be related to an augmented sensitivity of PFC neurons to orally consumed alcohol.

The reinforcing effects of ethanol within the posterior ventral tegmental area depend on dopamine neurotransmission to forebrain cortico-limbic systems

Ethanol can be self-infused directly into the posterior ventral tegmental area (pVTA) and these effects involve activation of local dopamine neurons. However, the neuro-circuitry beyond the pVTA involved in these reinforcing effects has not been explored. Intra-pVTA microinjection of ethanol increases dopamine release in the nucleus accumbens (NAC), medial prefrontal cortex (mPFC) and ventral pallidum (VP). The present study tested the hypothesis that the reinforcing effects of ethanol within the pVTA involve the activation of dopamine projections from the pVTA to the NAC, VP and mPFC. Following the acquisition of self-infusions of 200 mg% ethanol into the pVTA, either the dopamine D2 receptor antagonist sulpiride (0, 10 or 100 μM) or the D1 receptor antagonist SCH-23390 (0, 10 or 100 μM) was microinjected into the ipsilateral NAC shell (NACsh), NAC core (NACcr), VP or mPFC immediately prior to the self-infusion sessions to assess the involvement of the different dopamine projections in the reinforcing effects of ethanol. Microinjection of each compound at higher concentration into the NACsh, VP or mPFC, but not the NACcr, significantly reduced the responses on the active lever (from 40-50 to approximately 20 responses). These results indicate that activation of dopamine receptors in the NACsh, VP or mPFC, but not the NACcr, is involved in mediating the reinforcing effects of ethanol in the pVTA, suggesting that the 'alcohol reward' neuro-circuitry consist of, at least in part, activation of the dopamine projections from the pVTA to the NACsh, VP and mPFC.

Cell-based reporters reveal in vivo dynamics of dopamine and norepinephrine release in murine cortex

The neuronal coding of stimulus-to-action sequences is believed to involve the release of dopamine (DA) and norepinephrine (NE). The electrochemical similarity of these monoamines, however, confounds real-time measurements of their release. Here we report cell-based neurotransmitter fluorescent engineered reporters (CNiFERs) that use the specificity of G protein-coupled receptors (GPCRs) to discriminate nanomolar concentrations of DA and NE. CNiFERs were implanted into the frontal cortex of mice to measure the timing of neurotransmitter release during classical conditioning with the use of two-photon microscopy. The onset of DA release correlated with that of licking and shifted from the time of the reward toward that of the cue upon conditioning. In contrast, concurrent release of NE did not correlate with licking or the cue. This generation of CNiFERs provides unique tools to assess the release of monoamines. The molecular design of these CNiFERs may be generalized to realize CNiFERs for any molecule that activates a GPCR.

Translational clinical neuroscience perspectives on the cognitive and neurobiological mechanisms underlying alcohol-related aggression

Alcohol-related violence, a longstanding, serious, and pervasive social problem, has provided researchers from diverse disciplines with a model to study individual differences in aggressive and violent behavior. Of course, not all alcohol consumers will become aggressive after drinking and similarly, not all individuals with alcohol use disorders will exhibit such untoward behavior. Rather, the relationship is best conceptualized as complex and indirect and is influenced by a constellation of social, cognitive, and biological factors that differ greatly from one person to the next. Animal experiments and human studies have elucidated how these mechanisms and processes explain (i.e., mediate) the relation between acute and chronic alcohol consumption and aggressive behavior. Further, the rich body of literature on alcohol-related aggression has allowed for identification of several potential high-yield targets for clinical intervention, e.g., cognitive training for executive dysfunction; psychopharmacology targeting affect and threat perception, which may also generalize to other psychiatric conditions characterized by aggressive behavior. Here we aim to integrate pertinent findings, derived from different methodological approaches and theoretical models, which explain heterogeneity in aggressive responses to alcohol. A translational platform is provided, highlighting common factors linking alcohol and aggression that warrant further, interdisciplinary study in order to reduce the devastating social impact of this phenomenon.

Tolcapone suppresses ethanol intake in alcohol-preferring rats performing a novel cued access protocol

BACKGROUND

Dopamine (DA) has been shown to play a central role in regulating motivated behavior and encoding reward. Chronic drug abuse elicits a state of hypodopaminergia in the mesocorticolimbic (MCL) system in both humans and preclinical rodent models of addiction, including those modeling alcohol use disorders (AUD).

METHODS

Working under the hypothesis that reductions in the bioavailability of DA play an integral role in the expression of the excessive drinking phenotype, the catechol-O-methyltransferase (COMT) inhibitor tolcapone was used as a means to amplify cortical DA concentration and drinking behaviors were then assessed. Sucrose and ethanol (EtOH) consumption were measured in P and Wistar rats in both a free choice drinking protocol and a novel cued access protocol.

RESULTS

Tolcapone attenuated the consumption of EtOH, and to a lesser extent sucrose, in P rats in the cued access protocol, while no effect was observed in the free choice drinking protocol. Tolcapone also decreased EtOH consumption in high drinking Wistar rats. A follow-up experiment using the indirect DA agonist d-amphetamine showed no change in EtOH consumption.

CONCLUSIONS

Collectively, these data suggest that COMT inhibitors may be capable of alleviating the extremely motivating or salient nature of stimuli associated with alcohol. The hypothesis is put forth that the relative specificity of tolcapone for cortical DA systems may mediate the suppression of the high seeking/drinking phenotype.

Reinforcing properties and neurochemical response of ethanol within the posterior ventral tegmental area are enhanced in adulthood by periadolescent ethanol consumption

Alcohol drinking during adolescence is associated with increased alcohol drinking and alcohol dependence in adulthood. Research examining the biologic consequences of adolescent ethanol (EtOH) consumption on the response to EtOH in the neurocircuitry shown to regulate drug reinforcement is limited. The experiments were designed to determine the effects of periadolescent alcohol drinking on the reinforcing properties of EtOH within the posterior ventral tegmental area (pVTA) and the ability of EtOH microinjected into the pVTA to stimulate dopamine (DA) release in the nucleus accumbens shell (AcbSh). EtOH access (24-hour free-choice) by alcohol-preferring rats occurred during postnatal days (PND) 30-60. Animals were tested for their response to EtOH after PND 85. Intracranial self-administration techniques were performed to assess EtOH self-infusion into the pVTA. In the second experiment, rats received microinjections of EtOH into the pVTA, and dialysis samples were collected from the AcbSh. The results indicate that in rats that consumed EtOH during adolescence, the pVTA was more sensitive to the reinforcing effects of EtOH (a lower concentration of EtOH supported self-administration) and the ability of EtOH microinjected into the pVTA to stimulate DA release in the AcbSh was enhanced (sensitivity and magnitude). The data indicate that EtOH consumption during adolescence altered the mesolimbic DA system to be more sensitive and responsive to EtOH. This increase in the response to EtOH within the mesolimbic DA during adulthood could be part of biologic sequelae that are the basis for the deleterious effects of adolescent alcohol consumption on the rate of alcoholism during adulthood.

Decreased prefrontal cortical dopamine transmission in alcoholism

OBJECTIVE

Basic studies have demonstrated that optimal levels of prefrontal cortical dopamine are critical to various executive functions such as working memory, attention, inhibitory control, and risk/reward decisions, all of which are impaired in addictive disorders such as alcoholism. Based on this and imaging studies of alcoholism that have demonstrated less dopamine in the striatum, the authors hypothesized decreased dopamine transmission in the prefrontal cortex in persons with alcohol dependence.

METHOD

To test this hypothesis, amphetamine and [11C]FLB 457 positron emission tomography were used to measure cortical dopamine transmission in 21 recently abstinent persons with alcohol dependence and 21 matched healthy comparison subjects. [11C]FLB 457 binding potential, specific compared to nondisplaceable uptake (BPND), was measured in subjects with kinetic analysis using the arterial input function both before and after 0.5 mg kg-1 of d-amphetamine.

RESULTS

Amphetamine-induced displacement of [11C]FLB 457 binding potential (ΔBPND) was significantly smaller in the cortical regions in the alcohol-dependent group compared with the healthy comparison group. Cortical regions that demonstrated lower dopamine transmission in the alcohol-dependent group included the dorsolateral prefrontal cortex, medial prefrontal cortex, orbital frontal cortex, temporal cortex, and medial temporal lobe.

CONCLUSIONS

The results of this study, for the first time, unambiguously demonstrate decreased dopamine transmission in the cortex in alcoholism. Further research is necessary to understand the clinical relevance of decreased cortical dopamine as to whether it is related to impaired executive function, relapse, and outcome in alcoholism.

Negative urgency and ventromedial prefrontal cortex responses to alcohol cues: FMRI evidence of emotion-based impulsivity

BACKGROUND

Recent research has highlighted the role of emotion-based impulsivity (negative and positive urgency personality traits) for alcohol use and abuse, but has yet to examine how these personality traits interact with the brain's motivational systems. Using functional magnetic resonance imaging (fMRI), we tested whether urgency traits and mood induction affected medial prefrontal responses to alcohol odors (AcO).

METHODS

Twenty-seven social drinkers (mean age = 25.2, 14 males) had 6 fMRI scans while viewing negative, neutral, or positive mood images (3 mood conditions) during intermittent exposure to AcO and appetitive control (AppCo) aromas.

RESULTS

Voxel-wise analyses (p < 0.001) confirmed [AcO > AppCo] activation throughout medial prefrontal cortex (mPFC) and ventromedial PFC (vmPFC) regions. Extracted from a priori mPFC and vmPFC regions and analyzed in Odor (AcO, AppCo) × Mood factorial models, AcO activation was greater than AppCo in left vmPFC (p < 0.001), left mPFC (p = 0.002), and right vmPFC (p = 0.01) regions. Mood did not interact significantly with activation, but the covariate of trait negative urgency accounted for significant variance in left vmPFC (p = 0.01) and right vmPFC (p = 0.01) [AcO > AppCo] activation. Negative urgency also mediated the relationship between vmPFC activation and both (i) subjective craving and (ii) problematic drinking.

CONCLUSIONS

The trait of negative urgency is associated with neural responses to alcohol cues in the vmPFC, a region involved in reward value and emotion-guided decision-making. This suggests that negative urgency might alter subjective craving and brain regions involved in coding reward value.

Caffeinated Alcoholic Beverages - An Emerging Trend in Alcohol Abuse

Alcohol use disorders are pervasive in society and their impact affects quality of life, morbidity and mortality, as well as individual productivity. Alcohol has detrimental effects on an individual’s physiology and nervous system, and is associated with disorders of many organ and endocrine systems impacting an individual’s health, behavior, and ability to interact with others. Youth are particularly affected. Unfortunately, adolescent usage also increases the probability for a progression to dependence. Several areas of research indicate that the deleterious effects of alcohol abuse may be exacerbated by mixing caffeine with alcohol. Some behavioral evidence suggests that caffeine increases alcohol drinking and binge drinking episodes, which in turn can foster the development of alcohol dependence. As a relatively new public health concern, the epidemiological focus has been to establish a need for investigating the effects of caffeinated alcohol. While the trend of co-consuming these substances is growing, knowledge of the central mechanisms associated with caffeinated ethanol has been lacking. Research suggests that caffeine and ethanol can have additive or synergistic pharmacological actions and neuroadaptations, with the adenosine and dopamine systems in particular implicated. However, the limited literature on the central effects of caffeinated ethanol provides an impetus to increase our knowledge of the neuroadaptive effects of this combination and their impact on cognition and behavior. Research from our laboratories indicates that an established rodent animal model of alcoholism can be extended to investigate the acute and chronic effects of caffeinated ethanol.

Intravenous ethanol increases extracellular dopamine in the medial prefrontal cortex of the Long-Evans rat

BACKGROUND

Ethanol (EtOH) affects prefrontal cortex functional roles such as decision making, working memory, and behavioral control. Yet, the pharmacological effect of EtOH on dopamine, a neuromodulator in the medial prefrontal cortex (mPFC), is unclear. Past studies exploring this topic produced conflicting outcomes; however, a handful of factors (temporal resolution, method of drug administration, estrous cycle) possibly contributed to these discrepancies. We sought to mitigate these factors in order to elucidate EtOH's pharmacological effects on mPFC dopamine in Long-Evans rats.

METHODS

We administered experimental solutions via an intravenous (iv), handling-free route, monitored dopamine in the mPFC via microdialysis (10-minute samples), and used male rats to avoid estrous cycle/EtOH interactions. First, we rapidly (approximately 2.7 ml/min) or slowly (approximately 0.6 ml/min) administered 1.0 g/kg EtOH and saline infusions, showing that the experimental methods did not contribute to dopamine changes. Then, a cumulative dosing protocol was used to administer 0.25, 0.75, 1.50, and 2.25 g/kg iv EtOH doses to evaluate dose-response. Finally, we monitored dialysate EtOH levels during an oral EtOH self-administration session to compare the dialysate EtOH levels achieved during the pharmacological experiments to those seen during self-administration.

RESULTS

IV administration of a rapid or slow 1.0 g/kg EtOH infusion resulted in similar significant 55 ± 9 and 63 ± 15% peak dialysate dopamine increases, respectively. The 0.25, 0.75, 1.50, and 2.25 g/kg EtOH doses produced a nonsignificant 17 ± 5% and significant 36 ± 15, 68 ± 19, and 86 ± 20% peak dialysate dopamine increases, respectively. Self-administration dialysate EtOH concentrations fell within the range of concentrations noted during the EtOH dose-response curve.

CONCLUSIONS

These experiments show that, using experimental methods that minimize possibly confounding factors, acute iv EtOH increases extracellular dopamine in the mPFC in a dose-dependent manner, thereby clarifying EtOH's pharmacological effects on the mesocortical dopamine system.

Microinjections of acetaldehyde or salsolinol into the posterior ventral tegmental area increase dopamine release in the nucleus accumbens shell

BACKGROUND

Published findings indicate that acetaldehyde (ACD; the first metabolite of ethanol [EtOH]) and salsolinol (SAL; formed through the nonenzymatic condensation of ACD and dopamine [DA]) can be formed following EtOH consumption. Both ACD and SAL exhibit reinforcing properties within the posterior ventral tegmental area (pVTA) and both exhibit an inverted "U-shaped" dose-response curve. The current study was undertaken to examine the dose-response effects of microinjections of ACD or SAL into the pVTA on DA efflux in the nucleus accumbens shell (AcbSh).

METHODS

For the first experiment, separate groups of male Wistar rats received pulse microinjections of artificial cerebrospinal fluid (aCSF) or 12-, 23-, or 90-μM ACD into the pVTA, while extracellular DA levels were concurrently measured in the AcbSh. The second experiment was similarly conducted, except rats were given microinjections of aCSF or 0.03-, 0.3-, 1.0-, or 3.0-μM SAL, while extracellular levels of DA were measured in the AcbSh.

RESULTS

Both ACD and SAL produced a dose-dependent inverted "U-shaped" response on DA release in the AcbSh, with 23-μM ACD (200% baseline) and 0.3-μM SAL (300% baseline) producing maximal peak responses with higher concentrations of ACD (90 μM) and SAL (3.0 μM) producing significantly lower DA efflux.

CONCLUSIONS

The findings from the current study indicate that local application of intermediate concentrations of ACD and SAL stimulated DA neurons in the pVTA, whereas higher concentrations may be having secondary effects within the pVTA that inhibit DA neuronal activity. The present results parallel the studies on the reinforcing effects of ACD and SAL in the pVTA and support the idea that the reinforcing effects of ACD and SAL within the pVTA are mediated by activating DA neurons.

Repeated exposure of the posterior ventral tegmental area to nicotine increases the sensitivity of local dopamine neurons to the stimulating effects of ethanol

Clinical evidence indicates a frequent co-morbidity of nicotine and alcohol abuse and dependence. The posterior ventral tegmental area (pVTA) appears to support the reinforcing and dopamine-stimulating effects of both drugs. The current study tested the hypothesis that repeated exposure of the pVTA to one drug would increase the sensitivity of local dopamine neurons to the stimulating effects of the other drug. Female Wistar rats received repeated daily microinjections of either 100 μM nicotine or vehicle directly into the pVTA for 7 days. On the 8th day, rats received microinjections of either vehicle or ethanol (100 or 200 mg%) into the pVTA while extracellular dopamine samples were collected from the ipsilateral nucleus accumbens shell (NACsh) with microdialysis. Another experiment tested the effects of challenge microinjections of 200 μM nicotine in the pVTA on extracellular dopamine levels in the NACsh following 7 daily pretreatments with 200 mg% ethanol in the pVTA. Nicotine pretreatments increased the dopamine-stimulating effects of ethanol in the pVTA (100 mg% ethanol: 115% vs 160% of baseline in the vehicle and nicotine groups, respectively, p < 0.05; 200 mg% ethanol: 145% vs 190% of baseline in the vehicle and nicotine groups, respectively, p < 0.05). In contrast, ethanol pretreatments did not alter the stimulating effects of nicotine in the pVTA. The results suggest that repeated exposure of the pVTA to nicotine increased the response of local dopamine neurons to the stimulating effects of ethanol, whereas repeated exposure of the pVTA to ethanol did not alter the responses of pVTA dopamine neurons to nicotine.

Neurobiology of consummatory behavior: mechanisms underlying overeating and drug use

Consummatory behavior is driven by both caloric and emotional need, and a wide variety of animal models have been useful in research on the systems that drive consumption of food and drugs. Models have included selective breeding for a specific trait, manipulation of gene expression, forced or voluntary exposure to a substance, and identification of biomarkers that predict which animals are prone to overconsuming specific substances. This research has elucidated numerous brain areas and neurochemicals that drive consummatory behavior. Although energy homeostasis is primarily mediated by the hypothalamus, reinforcement is more strongly mediated by nuclei outside the hypothalamus, in mesocorticolimbic regions. Orexigenic neurochemicals that control food intake can provide a general signal for promoting caloric intake or a more specific signal for stimulating consumption of a particular macronutrient, fat, carbohydrate, or protein. The neurochemicals involved in controlling fat ingestion--galanin, enkephalin, orexin, melanin-concentrating hormone, and the endocannabinoids--show positive feedback with this macronutrient, as these peptides both increase fat intake and are further stimulated by its intake. This positive association offers some explanation for why foods high in fat are so often overconsumed. Consumption of ethanol, a drug of abuse that also contains calories, is similarly driven by the neurochemical systems involved in fat intake, according to evidence that closely relates fat and ethanol consumption. Further understanding of the systems involved in consummatory behavior will enable the development of effective therapies for the treatment of both overeating and drug abuse.

The stimulating effects of ethanol on ventral tegmental area dopamine neurons projecting to the ventral pallidum and medial prefrontal cortex in female Wistar rats: regional difference and involvement of serotonin-3 receptors

RATIONALE

The ventral tegmental area (VTA) mediates the local stimulating effects of ethanol (EtOH) in a region-dependent manner, with EtOH administration in the posterior but not anterior VTA stimulating the mesolimbic system. The serotonin-3 (5-HT(3)) receptor has been involved in the effects of EtOH on the mesolimbic system.

OBJECTIVES

The current study tested the hypothesis that EtOH would stimulate mesopallidal and mesocortical dopamine neurons in the posterior but not anterior VTA and that the stimulating effects of EtOH in the VTA would involve activation of local 5-HT(3) receptors.

METHODS

Wistar female rats were surgically implanted with two cannulae, one in one sub-region of the VTA for microinjection and the other in the ventral pallidum (VP) or medial prefrontal cortex (mPFC) for microdialysis. Artificial cerebrospinal fluid or EtOH (200 mg%; 44 mM) was microinjected in the anterior or posterior VTA, and extracellular dopamine was measured in the VP or mPFC with microdialysis-HPLC.

RESULTS

EtOH injections in the posterior but not anterior VTA significantly increased extracellular dopamine levels in the VP and mPFC. Co-injections of the 5-HT(3) receptor antagonist ICS-205,930 with EtOH in the posterior VTA significantly reduced the effects of EtOH on extracellular dopamine levels in the VP and mPFC.

CONCLUSIONS

The results indicate that posterior VTA dopamine neurons projecting to the VP and mPFC are stimulated by local administration of EtOH and that the local stimulating effects of EtOH are mediated, at least in part, by 5-HT(3) receptors.

Cognitive and neurobiological mechanisms of alcohol-related aggression

Alcohol-related violence is a serious and common social problem. Moreover, violent behaviour is much more common in alcohol-dependent individuals. Animal experiments and human studies have provided insights into the acute effect of alcohol on aggressive behaviour and into common factors underlying acute and chronic alcohol intake and aggression. These studies have shown that environmental factors, such as early-life stress, interact with genetic variations in serotonin-related genes that affect serotonergic and GABAergic neurotransmission. This leads to increased amygdala activity and impaired prefrontal function that, together, predispose to both increased alcohol intake and impulsive aggression. In addition, acute and chronic alcohol intake can further impair executive control and thereby facilitate aggressive behaviour.

What aspects of human alcohol use disorders can be modeled using selectively bred rat lines?

The use of selective breeding to produce animal models for the study of alcohol abuse and alcoholism represents one of the major advances in the field of alcohol research. Rats selectively bred for alcohol preference and alcohol nonpreference have been useful to both preclinical and clinical investigators in the alcohol research community for studying the behavioral, neurobiological, and molecular basis of alcohol drinking, for identifying the genes that may contribute to the development of alcohol abuse and alcoholism, and for evaluating the utility of drugs aimed at reducing alcohol intake and preventing alcohol relapse. Rats selectively bred for alcohol preference (alcohol preferring or "P" line) have enhanced responsiveness to the low dose reinforcing effects of alcohol, less aversion to moderate/high doses of alcohol, and are able to develop tolerance to the aversive effects of alcohol more rapidly and to maintain tolerance longer than rats selectively bred for alcohol nonpreference (alcohol nonpreferring or "NP" line). The increased potency of low-dose alcohol as a reinforcer for P rats might be expected to foster and maintain alcohol drinking. Weaker aversion to the pharmacological effects of moderate/high doses of alcohol in the P line would allow P rats to drink more alcohol than NP rats before the postingestional effects become aversive. Rapid induction of tolerance to the aversive effects of alcohol with repeated bouts of voluntary alcohol drinking, as well as persistence of alcohol tolerance in rats of the P line might serve to maintain alcohol drinking. These are powerful mechanisms that may serve to promote and maintain a high alcohol drinking behavior. Although these rat lines have been used to address several characteristics of excessive alcohol consumption in humans, they have not yet been used to model several aspects of human alcohol use disorders. New applications of these selectively bred rat lines are discussed which may further our understanding of the factors contributing to alcohol abuse and alcoholism.

Family history of alcoholism mediates the frontal response to alcoholic drink odors and alcohol in at-risk drinkers

Although a family history of alcoholism is the strongest risk factor for developing alcohol dependence, there are few studies of the association between familial alcoholism and the human brain's reward system activity. We used functional magnetic resonance imaging (fMRI) to determine how family history affects the brain's response to subjects' preferred alcoholic drink odors (AO) as compared to appetitive control odors (ApCO). Fourteen non-dependent heavy drinkers (HD) who were family history positive (FHP) participated, as did 12 HD who were family history negative (FHN). Subjects were imaged under both alcohol intoxication and placebo, using intravenous infusion and pharmacokinetic modeling to target a blood alcohol level of 50 mg%. Under placebo, HD-FHP had a larger medial frontal [AO>ApCO] effect than did HD-FHN. Alcohol intoxication dampened this response in the HD-FHP but potentiated it in the HD-FHN. This suggests that a family history of alcoholism and brain exposure to alcohol interact in heavy drinkers to differentially affect how the brain responds to alcohol cues.

Alcohol and the prefrontal cortex

The prefrontal cortex occupies the anterior portion of the frontal lobes and is thought to be one of the most complex anatomical and functional structures of the mammalian brain. Its major role is to integrate and interpret inputs from cortical and sub-cortical structures and use this information to develop purposeful responses that reflect both present and future circumstances. This includes both action-oriented sequences involved in obtaining rewards and inhibition of behaviors that pose undue risk or harm to the individual. Given the central role in initiating and regulating these often complex cognitive and behavioral responses, it is no surprise that alcohol has profound effects on the function of the prefrontal cortex. In this chapter, we review the basic anatomy and physiology of the prefrontal cortex and discuss what is known about the actions of alcohol on the function of this brain region. This includes a review of both the human and animal literature including information on the electrophysiological and behavioral effects that follow acute and chronic exposure to alcohol. The chapter concludes with a discussion of unanswered questions and areas needing further investigation.

Ethanol-induced hyperactivity is associated with hypodopaminergia in the 22-TNJ ENU-mutated mouse

Characterization of neurochemical and behavioral responses to ethanol in phenotypically distinct mouse strains can provide insight into the mechanisms of ethanol stimulant actions. Increases in striatal dopamine (DA) levels have often been linked to ethanol-induced hyperactivity. We examined the functional status of the DA system and behavioral responsiveness to ethanol, cocaine, and a DA-receptor agonist in an N-ethyl-N-nitrosourea-mutagenized mouse strain, 22-TNJ, generated by the Integrative Neuroscience Initiative on Alcoholism Consortium. The 22-TNJ mouse strain exhibited greater locomotor responses to 2.25g/kg ethanol and 10mg/kg cocaine, compared with control mice. In vivo microdialysis showed low-baseline DA levels and a larger DA increase with both 2.25g/kg ethanol and 10mg/kg cocaine. In in vitro voltammetry studies, the 22-TNJ mice displayed increased V(max) rates for DA uptake, possibly contributing to the low-baseline DA levels found with microdialysis. Finally, 22-TNJ mice showed enhanced in vitro autoreceptor sensitivity to the D2/D3 agonist, quinpirole, and greater locomotor responses to both autoreceptor-selective and postsynaptic receptor-selective doses of apomorphine compared with controls. Taken together, these results indicate that the dopaminergic system of the 22-TNJ mouse is low functioning compared with control, with consequent receptor supersensitivity, such that mutant animals exhibit enhanced behavioral responses to DA-activating drugs, such as ethanol. Thus, the 22-TNJ mouse represents a model for a relatively hypodopaminergic system, and could provide important insights into the mechanisms of hyper-responsiveness to ethanol's stimulant actions.

Sensitization of ventral tegmental area dopamine neurons to the stimulating effects of ethanol

BACKGROUND

Previous studies indicated that chronic alcohol drinking increased the sensitivity of the posterior ventral tegmental area (p-VTA) to the reinforcing effects of ethanol. The current study tested the hypothesis that local exposure of the p-VTA to ethanol would increase the sensitivity of dopamine (DA) neurons to the stimulating effects of ethanol.

METHODS

Experiment 1 examined the stimulating effects of ethanol in the p-VTA after a 7-day ethanol pretreatment in the p-VTA. Adult female Wistar rats were pretreated with microinjections of 200 mg% ethanol or artificial cerebrospinal fluid (aCSF) into the p-VTA once a day for 7 days. On the eighth day, rats received a challenge injection of ethanol (100, 200, or 300 mg%) or aCSF into the p-VTA, and extracellular DA levels were measured in the nucleus accumbens (NAc) shell with microdialysis. Experiment 2 examined the stimulating effects of ethanol (200 mg%) after a 3- or 5-day ethanol (200 mg%) pretreatment in the p-VTA. Experiment 3 examined the stimulating effects of ethanol (200 mg%) 7 days after the last of the 7-day ethanol (200 mg%) pretreatments in the p-VTA.

RESULTS

Experiment 1: in both aCSF- and ethanol-pretreated rats, the challenge microinjection of ethanol dose-dependently increased DA release in the NAc shell, with significantly greater increases in ethanol-pretreated groups. Experiment 2: the 5-day, but not 3-day, ethanol pretreatment protocol increased the response of p-VTA dopamine neurons to the ethanol challenge. Experiment 3: the increased stimulating effects of ethanol were still evident after 7 days.

CONCLUSIONS

The results indicate that repeated local ethanol exposure of the p-VTA produced neuroadaptations in DA neurons projecting to the NAc shell, resulting in a persistent increase in the sensitivity of these neurons to the stimulating effects of ethanol.