A single, moderate ethanol exposure alters extracellular dopamine levels and dopamine d receptor function in the nucleus accumbens of wistar rats

Comorbidity of Post-Traumatic Stress Disorder and Alcohol Use Disorder: Animal Models and Associated Neurocircuitry

Post-traumatic stress disorder (PTSD) and alcohol use disorder (AUD) are prevalent neuropsychiatric disorders and frequently co-occur concomitantly. Individuals suffering from this dual diagnosis often exhibit increased symptom severity and poorer treatment outcomes than those with only one of these diseases. Lacking standard preclinical models limited the exploration of neurobiological mechanisms underlying PTSD and AUD comorbidity. In this review, we summarize well-accepted preclinical model paradigms and criteria for developing successful models of comorbidity. We also outline how PTSD and AUD affect each other bidirectionally in the nervous nuclei have been heatedly discussed recently. We hope to provide potential recommendations for future research.

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.

Deletion of Dlk1 increases the vulnerability to developing anxiety-like behaviors and ethanol consumption in mice

Anxiety and alcohol use disorders (AUD) often present together, constituting a significant public health problem worldwide. In this study, we investigated the role of DLK1, a ligand of the Delta/NOTCH epidermal growth factor (EGF)-like protein family, reported to play a role in DA neurons differentiation in the striatum, as a neurobiological factor involved in the mechanisms regulating this psychiatric comorbidity. We exposed Dlk1 knockout mice (Dlk1-/- mice) to the open-field (OF), the light-dark box (LBD) and the elevated plus maze (EPM) tests, evaluating motivation to drink and ethanol consumption using the oral ethanol self-administration (OEA) paradigm. Quantitative real time polymerase chain reaction (qPCR) studies were carried out to evaluate alterations in targets closely related to DA neurotransmission in the reward system, tyrosine hydroxylase (Th) in the ventral tegmental area (VTA), and μ-opioid receptor (Oprm1) in the nucleus accumbens (NAc). No differences were observed in the total or peripheral distances travelled by Dlk1-/- compared to wild-type (WT) mice in OF. However, central distance travelled significantly decreased in Dlk1-/- mice. Deletion of Dlk1 increased anxiety-like behaviors in the LDB and EPM, and, Dlk1-/- mice also presented higher ethanol intake and motivation to drink (number of effective responses) in the OEA. In addition, Th and Oprm1 gene expression was reduced in the VTA and NAc of Dlk1-/- mice. We conclude that deletion of Dlk1 increases anxiety-related behaviors and vulnerability to ethanol consumption and modifies the gene expression of key targets closely related with DA neurotransmission involved in the reinforcing actions of ethanol.

Consumption of Alcopops During Brain Maturation Period: Higher Impact of Fructose Than Ethanol on Brain Metabolism

Alcopops are flavored alcoholic beverages sweetened by sodas, known to contain fructose. These drinks have the goal of democratizing alcohol among young consumers (12-17 years old) and in the past few years have been considered as fashionable amongst teenagers. Adolescence, however, is a key period for brain maturation, occurring in the prefrontal cortex and limbic system until 21 years old. Therefore, this drinking behavior has become a public health concern. Despite the extensive literature concerning the respective impacts of either fructose or ethanol on brain, the effects following joint consumption of these substrates remains unknown. Our objective was to study the early brain modifications induced by a combined diet of high fructose (20%) and moderate amount of alcohol in young rats by ¹³C Nuclear Magnetic Resonance (NMR) spectroscopy. Wistar rats had isocaloric pair-fed diets containing fructose (HF, 20%), ethanol (Et, 0.5 g/day/kg) or both substrates at the same time (HFEt). After 6 weeks of diet, the rats were infused with ¹³C-glucose and brain perchloric acid extracts were analyzed by NMR spectroscopy (¹H and ¹³C). Surprisingly, the most important modifications of brain metabolism were observed under fructose diet. Alterations, observed after only 6 weeks of diet, show that the brain is vulnerable at the metabolic level to fructose consumption during late-adolescence throughout adulthood in rats. The main result was an increase in oxidative metabolism compared to glycolysis, which may impact lactate levels in the brain and may, at least partially, explain memory impairment in teenagers consuming alcopops.

Exploring the consequences of social defeat stress and intermittent ethanol drinking on dopamine dynamics in the rat nucleus accumbens

The current study aimed to explore how presynaptic dopamine (DA) function is altered following brief stress episodes and chronic ethanol self-administration and whether these neuroadaptations modify the acute effects of ethanol on DA dynamics. We used fast-scan cyclic voltammetry to evaluate changes in DA release and uptake parameters in rat nucleus accumbens brain slices by analyzing DA transients evoked through single pulse electrical stimulation. Adult male rats were divided into four groups: ethanol-naïve or ethanol drinking (six week intermittent two-bottle choice) and stressed (mild social defeat) or nonstressed. Results revealed that the mild stress significantly increased DA release and uptake in ethanol-naïve subjects, compared to nonstressed controls. Chronic ethanol self-administration increased the DA uptake rate and occluded the effects of stress on DA release dynamics. Bath-applied ethanol decreased stimulated DA efflux in a concentration-dependent manner in all groups; however, the magnitude of this effect was blunted by either stress or chronic ethanol, or by a combination of both procedures. Together, these findings suggest that stress and ethanol drinking may promote similar adaptive changes in accumbal presynaptic DA release measures and that these changes may contribute to the escalation in ethanol intake that occurs during the development of alcohol use disorder.

Syntaxin 1B contributes to regulation of the dopaminergic system through GABA transmission in the CNS

In neuronal plasma membrane, two syntaxin isoforms, HPC-1/syntaxin 1A (STX1A) and syntaxin 1B (STX1B), are predominantly expressed as soluble N-ethylmaleimide-sensitive fusion attachment protein receptors, also known as t-SNAREs. We previously reported that glutamatergic and GABAergic synaptic transmissions are impaired in Stx1b null mutant (Stx1b^-/- ) mice but are almost normal in Stx1a null mutant (Stx1a^-/- ) mice. These observations suggested that STX1A and STX1B have distinct functions in fast synaptic transmission in the central nervous system (CNS). Interestingly, recent studies indicated that Stx1a^-/- or Stx1a^+/- mice exhibit disruption in the monoaminergic system in the CNS, causing unusual behaviour that is similar to neuropsychological alterations observed in psychiatric patients. Here, we studied whether STX1B contributes to the regulation of monoaminergic system and if STX1B is related to neuropsychological properties in human neuropsychological disorders similar to STX1A. We found that monoamine release in vitro was normal in Stx1b^+/- mice unlike Stx1a^-/- or Stx1a^+/- mice, but the basal extracellular dopamine (DA) concentration in the ventral striatum was increased. DA secretion in the ventral striatum is regulated by GABAergic neurons, and Stx1b^+/- mice exhibited reduced GABA release both in vitro and in vivo, disrupting the DAergic system in the CNS of these mice. We also found that Stx1b^+/- mice exhibited reduced pre-pulse inhibition (PPI), which is believed to represent one of the prominent schizotypal behavioural profiles of human psychiatric patients. The reduction in PPI was rescued by DA receptor antagonists. These observations indicated that STX1B contributes to excess activity of the DAergic system through regulation of GABAergic transmission.

Initial subjective reward to alcohol in Sprague-Dawley rats

Initial subjective response to the rewarding properties of alcohol predicts voluntary consumption and the risk for alcohol use disorders. We assessed the initial subjective reward to alcohol in rats using a single exposure conditioned place preference (SE-CPP) paradigm. Sprague-Dawley rats demonstrate preference for a context paired with a single systemic injection of ethanol (1.0 g/kg, delivered intraperitoneally). However, expression of SE-CPP in males depended on pairing ethanol with the first exposure of two (ethanol; saline) to the conditioning apparatus and procedures, while conditioning day did not appreciably affect SE-CPP in females, consistent with the view that females experience heightened addiction vulnerability. This model offers researchers a high throughput assay for investigating factors that influence alcohol reward and may point the way toward more effective prevention and treatment efforts.

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.

Stereological analyses of reward system nuclei in maternally deprived/separated alcohol drinking rats

The experience of early life stress can trigger complex neurochemical cascades that influence emotional and addictive behaviors later in life in both adolescents and adults. Recent evidence suggests that excessive alcohol drinking and drug-seeking behavior, in general, are co-morbid with depressive-like behavior. Both behaviors are reported in humans exposed to early life adversity, and are prominent features recapitulated in animal models of early life stress (ELS) exposure. Currently, little is known about whether or how ELS modulates reward system nuclei. In this study we use operant conditioning of rats to show that the maternal separation stress (MS) model of ELS consumes up to 3-fold greater quantities of 10% vol/vol EtOH in 1-h, consistently over a 3-week period. This was correlated with a significant 22% reduction in the number of dopaminergic-like neurons in the VTA of naïve MS rats, similar to genetically alcohol-preferring (P) rats which show a 35% reduction in tyrosine hydroxylase (TH)-positive dopaminergic neurons in the VTA. MS rats had a significantly higher 2-fold immobility time in the forced swim test (FST) and reduced sucrose drinking compared to controls, indicative of depressive-like symptomology and anhedonia. Consistent with this finding, stereological analysis revealed that amygdala neurons were 25% greater in number at P70 following MS exposure. Our previous examination of the dentate gyrus of hippocampus, a region involved in encoding emotional memory, revealed fewer dentate gyrus neurons after MS, but we now report this reduction in neurons occurs without effect on the number of astrocytes or length of astrocytic fibers. These data indicate that MS animals exhibit neuroanatomical changes in reward centers similar to those reported for high alcohol drinking rats, but aspects of astrocyte morphometry remained unchanged. These data are of high relevance to understand the breadth of neuronal pathology that ensues in reward loci following ELS.

Involvement of dopamine D2 receptors in addictive-like behaviour for acetaldehyde

Acetaldehyde, the first metabolite of ethanol, is active in the central nervous system, where it exerts motivational properties. Acetaldehyde is able to induce drinking behaviour in operant-conflict paradigms that resemble the core features of the addictive phenotype: drug-intake acquisition and maintenance, drug-seeking, relapse and drug use despite negative consequences. Since acetaldehyde directly stimulates dopamine neuronal firing in the mesolimbic system, the aim of this study was the investigation of dopamine D2-receptors' role in the onset of the operant drinking behaviour for acetaldehyde in different functional stages, by the administration of two different D2-receptor agonists, quinpirole and ropinirole. Our results show that acetaldehyde was able to induce and maintain a drug-taking behaviour, displaying an escalation during training, and a reinstatement behaviour after 1-week forced abstinence. Acetaldehyde operant drinking behaviour involved D2-receptor signalling: in particular, quinpirole administration at 0.03 mg/kg, induced a significant decrease in the number of lever presses both in extinction and in relapse. Ropinirole, administered at 0.03 mg/kg during extinction, did not produce any modification but, when administered during abstinence, induced a strong decrease in acetaldehyde intake in the following relapse session. Taken together, our data suggest that acetaldehyde exerts its own motivational properties, involving the dopaminergic transmission: indeed, activation of pre-synaptic D2-receptors by quinpirole, during extinction and relapse, negatively affects operant behaviour for acetaldehyde, likely decreasing acetaldehyde-induced dopamine release. The activation of post-synaptic D2-receptors by ropinirole, during abstinence, decreases the motivation to the consecutive reinstatement of acetaldehyde drinking behaviour, likely counteracting the reduction in the dopaminergic tone typical of withdrawal. These data further strengthen the evidence that acetaldehyde may play a crucial role as mediator of ethanol's central effects.

Involvement of D1/D2 dopamine receptors within the nucleus accumbens and ventral tegmental area in the development of sensitization to antinociceptive effect of morphine

The nucleus accumbens (NAc) and the ventral tegmental area (VTA) are two major areas for the mesolimbic dopaminergic system which are strongly involved in the development of behavioral sensitization. In the present study, we investigated the role of D1/D2 dopaminergic receptors within the NAc or VTA in response to sensitization to morphine by the tail-flick test as a model of acute pain. Sensitization was induced by subcutaneous (SC) injection of morphine (5 mg/kg), once daily for three days followed by 5 days free of drug. After the sensitization period, antinociceptive responses induced by an ineffective dose of morphine (1 mg/kg; SC) were obtained by the tail-flick test, and represented as maximal possible effect (%MPE). In experimental groups, D1 and D2 receptor antagonists, SCH-23390 and sulpiride (0.25, 1 and 4 μg/rat), were separately microinjected into the NAc or VTA, 10 min before morphine administration during the sensitization period, respectively. Results showed that injection of morphine during the sensitization period (development of sensitization) increased %MPE of the ineffective dose of morphine from 2.43±1.4% in naive to 47.75±4.01% in sensitized animals (P<0.001). Unilateral microinjections of different doses of the D1/D2 receptor antagonists, SCH-23390 and sulpiride, into the NAc dose-dependently decreased %MPEs in morphine-sensitized animals. Nonetheless, %MPEs were only affected by intra-VTA administration of SCH-23390 in morphine-sensitized animals (P<0.05). Our findings suggest that both the D1/D2 dopamine receptors in the NAc and the D1 receptors in the VTA may be of more important in the development of sensitization to morphine in rats.

β-Arrestin 2 knockout mice exhibit sensitized dopamine release and increased reward in response to a low dose of alcohol

RATIONALE

The rewarding effects of alcohol have been attributed to interactions between opioid and dopaminergic system within the mesolimbic reward pathway. We have previously shown that ablation of β-arrestin 2 (Arrb2), a crucial regulator of μ-opioid receptor function, attenuates alcohol-induced hyperlocomotion and c-fos activation in the nucleus accumbens.

OBJECTIVES

Here, we further investigated the role of Arrb2 in modulating alcohol-induced dopamine (DA) release and conditioned place preference (CPP). We also assessed the functional importance of Arrb2 for μ-opioid receptor surface expression and signaling following an acute alcohol challenge.

METHODS

Alcohol-evoked (0.375, 0.75, and 1.5 g/kg intraperitoneally) DA release was measured by in vivo microdialysis in the shell of nucleus accumbens. Reward was assessed by the CPP paradigm. Receptor function was assessed by μ-receptor binding and [(35)S]GTP-γ-S autoradiography.

RESULTS

In Arrb2 knockout mice accumbal DA levels reach maximum response at a lower dose compared to wild-type (wt) animals. In line with these results, Arrb2 knockout mice display increased CPP for alcohol as compared to wt mice. Finally, Arrb2 mutant mice display increased μ-opioid receptor signaling in the ventral and dorsal striatum and amygdala in response to a low dose of alcohol, indicating impaired desensitization mechanisms in these mice.

CONCLUSIONS

Our results show that Arrb2 modulates the response to low doses of alcohol on various levels including μ-opioid receptor signaling, DA release, and reward. They also reveal a clear dissociation between the effects of Arrb2 on psychomotor and reward behaviors.

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.

Introduction to the special issue "Pharmacotherapies for the treatment of alcohol abuse and dependence" and a summary of patents targeting other neurotransmitter systems

This paper introduces the Special Section: Pharmacotherapies for the Treatment of Alcohol Abuse and Dependence and provides a summary of patents targeting neurotransmitter systems not covered in the other four chapters. The World Health Organization notes that alcoholic-type drinking results in 2.5 million deaths per year, and these deaths occur to a disproportionately greater extent among adolescents and young adults. Developing a pharmacological treatment targeting alcohol abuse and dependence is complicated by (a) the heterogeneous nature of the disease(s), (b) alcohol affecting multiple neurotransmitter and neuromodulator systems, and (c) alcohol affecting multiple organ systems which in turn influence the function of the central nervous system. Presently, the USA Federal Drug Administration has approved three pharmacotherapies for alcoholism: disulfiram, naltrexone, and acamprosate. This chapter provides a summary of the following systems, which are not covered in the accompanying chapters; alcohol and acetaldehyde metabolism, opioid, glycinergic, GABA-A, neurosteroid, dopaminergic, serotonergic, and endocannabinoid, as well as patents targeting these systems for the treatment of alcoholism. Finally, an overview is presented on the use of pharmacogenetics and pharmacogenomics in tailoring treatments for certain subpopulations of alcoholics, which is expected to continue in the future.

Decreased sensitivity of NMDA receptors on dopaminergic neurons from the posterior ventral tegmental area following chronic nondependent alcohol consumption

BACKGROUND

The mesocorticolimbic dopamine system mediates the reinforcing effects of salient stimuli, including drugs of abuse. Nondependent chronic alcohol consumption modifies this system, resulting in an increased number of spontaneously active dopamine neurons in the posterior ventral tegmental area (VTA) of alcohol-preferring (P) rats. Enhanced responses of postsynaptic glutamate receptors may contribute to the increase in active dopamine neurons. Thus, excitations of putative dopamine neurons to locally applied N-methyl-d-aspartic acid (NMDA; glutamate receptor subtype agonist) were evaluated.

METHODS

P rats were assigned to alcohol naïve (water only) or alcohol drinking (continuous access to 15% alcohol and water for 8 consecutive weeks) groups. Responses of 23 putative dopamine neurons from naïve rats and 19 putative dopamine neurons from drinking rats were assessed in vivo using microiontophoretically applied NMDA. Current-response curves for firing frequency and burst activity were constructed using nonlinear mixed effects models. Between-group comparisons were made for EC(50) (effective current producing a half maximal excitatory response), E(max) (maximal excitatory effect), and C(DB) (the current at which depolarization block-marked decrease in neuronal activity-occurred).

RESULTS

Drinking P rats steadily consumed alcohol over the 8-week protocol and did not exhibit signs of dependence or withdrawal. Putative dopamine neurons from drinking rats exhibited resistance to depolarization block (higher C(DB) values) and required larger doses of NMDA to elicit moderate excitatory responses (higher EC(50) values), consistent with decreased receptor affinity. Maximal excitatory responses (E(max) ) did not differ between the groups, consistent with no change in receptor number. Blood alcohol was at undetectable levels at the time of experimentation.

CONCLUSIONS

NMDA receptor sensitivity is decreased on posterior VTA putative dopamine neurons in P rats on a nondependent schedule of alcohol consumption. Mechanisms underlying increased spontaneous dopamine neuron activity may be independent of changes in NMDA receptor function. Decreased NMDA receptor sensitivity may precede the development of dependence.

Subjective and neural responses to intravenous alcohol in young adults with light and heavy drinking patterns

Heavy alcohol consumption during young adulthood is a risk factor for the development of serious alcohol use disorders. Research has shown that individual differences in subjective responses to alcohol may affect individuals' vulnerability to developing alcoholism. Studies comparing the subjective and objective response to alcohol between light and heavy drinkers (HDs), however, have yielded inconsistent results, and neural responses to alcohol in these groups have not been characterized. We performed a double-blind, placebo-controlled, randomized crossover alcohol challenge study comparing functional magnetic resonance imaging and subjective response to intravenously administered 6% v/v ethanol to a target blood alcohol concentration of 0.08% or placebo between HDs and social drinkers (SDs). During the imaging, we presented emotional cues in order to measure how emotion modulated the effects of alcohol on the brain's reward circuitry. We found that, at equivalent blood alcohol concentrations, HDs reported lower subjective alcohol effects than SDs. Alcohol significantly activated the nucleus accumbens in SDs, but not in HDs. Self-reported ratings of intoxication correlated with striatal activation, suggesting that activation may reflect subjective experience of intoxication. Fearful faces significantly activated the amygdala in the SDs only, and this activation was attenuated by alcohol. This study shows that HDs not only experience reduced subjective effects of alcohol, but also demonstrate a blunted response to alcohol in the brain's reward system. Our findings indicate that reduced subjective and neural response to alcohol in HDs may be suggestive of either the development of tolerance to alcohol, or of pre-existing decreased sensitivity to alcohol's effects.

Accumbal dopamine D2 receptor function is associated with individual variability in ethanol behavioral sensitization

Striatal dopamine D2 receptors have been implicated in the development of behavioral sensitization after repeated exposure to drugs of abuse. There are clear individual differences in the level of sensitization to ethanol among species and even among individuals from the same strain. Albino Swiss mice treated with ethanol (2.2 g/kg) have been shown to present clear variations in the development of sensitization. While some mice developed ethanol (EtOH) induced sensitization, others did not. This variability was associated with differences in D2 dopaminergic receptor binding. In the present study, we evaluated the functional relevance of dopamine D2 receptor by measuring, in sensitized and non-sensitized mice, the locomotor response to a D2 receptor agonist (quinpirole, 0.5 and 2.0 mg/kg i.p. or 0.01 and 0.2 μg/side intra-accumbens) or antagonist (sulpiride, 10 or 50 mg/kg i.p. or 0.02 μg/side intra-accumbens + ethanol i.p.). Whereas the systemic administration of quinpirole decreased locomotor activity in a similar way in all the groups, intra-nucleus accumbens (NAc) administration induced significantly higher locomotor stimulation in the sensitized group alone. Our data show that functionally hyperresponsive D2 receptors are present in the NAcs of sensitized but not non-sensitized mice, suggesting that this could be a biomarker of behavioral sensitization. Furthermore, i.p. administration of sulpiride blocked the expression of sensitization in the sensitized group, and intra-NAc administration attenuated it, indicating that the activation of accumbal D2 receptors is essential for the expression of EtOH behavioral sensitization. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Ethanol drinking reduces extracellular dopamine levels in the posterior ventral tegmental area of nondependent alcohol-preferring rats

Moderate ethanol exposure produces neuroadaptive changes in the mesocorticolimbic dopamine (DA) system in nondependent rats and increases measures of DA neuronal activity in vitro and in vivo. Moreover, moderate ethanol drinking and moderate systemic exposure elevates extracellular DA levels in mesocorticolimbic projection regions. However, the neuroadaptive changes subsequent to moderate ethanol drinking on basal DA levels have not been investigated in the ventral tegmental area (VTA). In the present study, adult female alcohol-preferring (P) rats were divided into alcohol-naive, alcohol-drinking, and alcohol-deprived groups. The alcohol-drinking group had continuous access to water and ethanol (15%, vol/vol) for 8 weeks. The alcohol-deprived group had 6 weeks of access followed by 2 weeks of ethanol deprivation, 2 weeks of ethanol re-exposure, followed again by 2 weeks of deprivation. The deprived rats demonstrated a robust alcohol deprivation effect (ADE) on ethanol reinstatement. The alcohol-naïve group had continuous access to water only. In the last week of the drinking protocol, all rats were implanted with unilateral microdialysis probes aimed at the posterior VTA and no-net-flux microdialysis was conducted to quantify extracellular DA levels and DA clearance. Results yielded significantly lower basal extracellular DA concentrations in the posterior VTA of the alcohol-drinking group compared with the alcohol-naive and alcohol-deprived groups (3.8±0.3nM vs. 5.0±0.5nM [P<.02] and 4.8±0.4nM, [P<.05], respectively). Extraction fractions were significantly (P<.0002) different between the alcohol-drinking and alcohol-naive groups (72±2% vs. 46±4%, respectively) and not significantly different (P=.051) between alcohol-deprived and alcohol-naive groups (61±6% for the alcohol-deprived group). The data indicate that reductions in basal DA levels within the posterior VTA occur after moderate chronic ethanol intake in nondependent P rats. This reduction may result, in part, from increased DA uptake and may be important for the maintenance of ethanol drinking. These adaptations normalize with ethanol deprivation and may not contribute to the ADE.

Ventral and dorsal striatal dopamine efflux and behavior in rats with simple vs. co-morbid histories of cocaine sensitization and neonatal ventral hippocampal lesions

RATIONAL

Exposing animal models of mental illness to addictive drugs provides an approach to understanding the neural etiology of dual diagnosis disorders. Previous studies have shown that neonatal ventral hippocampal lesions (NVHL) in rats produce features of both schizophrenia and addiction vulnerability.

OBJECTIVE

This study investigated ventral and dorsal striatal dopamine (DA) efflux in NVHL rats combined with behavioral sensitization to cocaine.

METHODS

Adult NVHL vs. SHAM-operated rats underwent a 5-day injection series of cocaine (15 mg/kg/day) vs. saline. One week later, rats were cannulated in nucleus accumbens SHELL, CORE, or caudate-putamen. Another week later, in vivo microdialysis sampled DA during locomotor testing in which a single cocaine injection (15 mg/kg) was delivered.

RESULTS

NVHLs and cocaine history significantly increased behavioral activation approximately 2-fold over SHAM-saline history rats. DA efflux curves corresponded time dependently with the cocaine injection and locomotor curves and varied significantly by striatal region: Baseline DA levels increased 5-fold while cocaine-stimulated DA efflux decreased by half across a ventral to dorsal striatal gradient. However, NVHLs, prior cocaine history, and individual differences in behavior were not underpinned by differential DA efflux overall or within any striatal region.

CONCLUSION

Differences in ventral/dorsal striatal DA efflux are not present in and are not required for producing differential levels of acute cocaine-induced behavioral activation in NVHLs with and without a behaviorally sensitizing cocaine history. These findings suggest other neurotransmitter systems, and alterations in striatal network function post-synaptic to DA transmission are more important to understanding the interactive effects of addictive drugs and mental illness.