Ethanol modulates evoked dopamine release in mouse nucleus accumbens: dependence on social stress and dose

Friend of the Devil: Negative Social Influences Driving Substance Use Disorders

Substance use disorders in humans have significant social influences, both positive and negative. While prosocial behaviors promote group cooperation and are naturally rewarding, distressing social encounters, such as aggression exhibited by a conspecific, are aversive and can enhance the sensitivity to rewarding substances, promote the acquisition of drug-taking, and reinstate drug-seeking. On the other hand, withdrawal and prolonged abstinence from drugs of abuse can promote social avoidance and suppress social motivation, accentuating drug cravings and facilitating relapse. Understanding how complex social states and experiences modulate drug-seeking behaviors as well as the underlying circuit dynamics, such as those interacting with mesolimbic reward systems, will greatly facilitate progress on understanding triggers of drug use, drug relapse and the chronicity of substance use disorders. Here we discuss some of the common circuit mechanisms underlying social and addictive behaviors that may underlie their antagonistic functions. We also highlight key neurochemicals involved in social influences over addiction that are frequently identified in comorbid psychiatric conditions. Finally, we integrate these data with recent findings on (±)3,4-methylenedioxymethamphetamine (MDMA) that suggest functional segregation and convergence of social and reward circuits that may be relevant to substance use disorder treatment through the competitive nature of these two types of reward. More studies focused on the relationship between social behavior and addictive behavior we hope will spur the development of treatment strategies aimed at breaking vicious addiction cycles.

The Neural Processes Interlinking Social Isolation, Social Support, and Problem Alcohol Use

BACKGROUND

Subjective feeling of social isolation, as can be measured by perceived burdensomeness (PB), is a major risk factor for alcohol misuse. Heightened PB is associated with elevated stress response and diminished cognitive control, both of which contribute to problem drinking. Here, we sought to identify the neural substrates underlying the relationship between PB and alcohol misuse.

METHODS

We employed resting-state functional magnetic resonance imaging data collected from 61 problem drinkers to characterize the functional connectivity of the hypothalamus and ventral striatum (VS) in relation to PB. We specifically examined whether the connectivities of the hypothalamus and VS were differentially influenced by PB to produce contrasting effects on alcohol use. Finally, we evaluated how individual differences in social support modulate the inter-relationships of social isolation, neural connectivity, and the severity of problem drinking.

RESULTS

Whole-brain multiple regressions show a positive relationship between PB and hypothalamic connectivity with the hippocampus and an inverse pattern for VS connectivity with the middle frontal gyrus. Difference in strength between the 2 connectivities predicted the severity of problem drinking, suggesting an imbalance involving elevated hypothalamic and diminished prefrontal cortical modulation in socially isolated problem drinkers. A path analysis further revealed that the lack of social support was associated with a bias toward low prefrontal connectivity, which in turn increased PB and facilitated problem drinking.

CONCLUSIONS

Altered hypothalamus and VS connectivity may underlie problem drinking induced by social isolation. The current findings also highlight the important role of social support as a potential protective factor against alcohol misuse.

Social defeat stress and escalation of cocaine and alcohol consumption: Focus on CRF

Both the ostensibly aversive effects of unpredictable episodes of social stress and the intensely rewarding effects of drugs of abuse activate the mesocorticolimbic dopamine systems. Significant neuroadaptations in interacting stress and reward neurocircuitry may underlie the striking connection between stress and substance use disorders. In rodent models, recurring intermittent exposure to social defeat stress appears to produce a distinct profile of neuroadaptations that translates most readily to the repercussions of social stress in humans. In the present review, preclinical rodent models of social defeat stress and subsequent alcohol, cocaine or opioid consumption are discussed with regard to: (1) the temporal pattern of social defeat stress, (2) male and female protocols of social stress-escalated drug consumption, and (3) the neuroplastic effects of social stress, which may contribute to escalated drug-taking. Neuroadaptations in corticotropin-releasing factor (CRF) and CRF modulation of monoamines in the ventral tegmental area and the bed nucleus of the stria terminalis are highlighted as potential mechanisms underlying stress-escalated drug consumption. However, the specific mechanisms that drive CRF-mediated increases in dopamine require additional investigation as do the stress-induced neuroadaptations that may contribute to the development of compulsive patterns of drug-taking.

Persistent escalation of alcohol consumption by mice exposed to brief episodes of social defeat stress: suppression by CRF-R1 antagonism

RATIONALE

Episodic bouts of social stress can precede the initiation, escalation, or relapse to disordered alcohol intake. Social stress may engender neuroadaptations in the hypothalamic-pituitary-adrenal (HPA) axis and in extrahypothalamic stress circuitry to promote the escalation of alcohol intake.

OBJECTIVES

We aimed to (1) confirm a pattern of escalated drinking in socially defeated mice and to (2) test drugs that target distinct aspects of the HPA axis and extrahypothalamic neural substrates for their effectiveness in reducing murine, stress-escalated drinking.

METHODS

Male C57BL/6J (B6) mice were socially defeated by resident Swiss-derived males for ten consecutive days receiving 30 bites/day. Ten days after the final defeat, cohorts of B6 mice received continuous or intermittent access to 20% EtOH (w/v) and water. After 4 weeks of drinking, mice were injected with weekly, systemic doses of the CRF-R1 antagonist, CP376395; the glucocorticoid receptor antagonist, mifepristone; the 11-beta-hydroxylase inhibitor, metyrapone; or the 5-alpha-reductase inhibitor, finasteride.

RESULTS

Prior to drug treatments, defeated mice reliably consumed more EtOH than non-defeated controls, and mice given alcohol intermittently consumed more EtOH than those with continuous access. CP376395 (17-30 mg/kg) reduced continuous, but not intermittent EtOH intake (g/kg) in socially defeated mice. Mifepristone (100 mg/kg), however, increased drinking by defeated mice with intermittent access to alcohol while reducing drinking during continuous access. When administered finasteride (100 mg/kg) or metyrapone (50 mg/kg), all mice reduced their EtOH intake while increasing their water consumption.

CONCLUSIONS

Mice with a history of episodic social defeat stress were selectively sensitive to the effects of CRF-R1 antagonism, suggesting that CRF-R1 may be a potential target for treating alcohol use disorders in individuals who escalate their drinking after exposure to repeated bouts of psychosocial stress. Future studies will clarify how social defeat stress may alter the expression of extrahypothalamic CRF-R1 and glucocorticoid receptors.

Differential effects of acute administration of SCH-23390, a D₁ receptor antagonist, and of ethanol on swimming activity, anxiety-related responses, and neurochemistry of zebrafish

RATIONALE

The zebrafish has become an increasingly popular animal model for investigating ethanol's actions in the brain and its effects on behavior. Acute exposure to ethanol in zebrafish has been shown to induce a dose-dependent increase of locomotor activity, to reduce fear- and anxiety-related behavioral responses, and to increase the levels of dopamine and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC).

OBJECTIVES

The objective of the present study was to investigate the role of dopamine D1 receptors (D1-R) in ethanol-induced locomotor activity in zebrafish.

METHODS

Zebrafish were pre-treated with SCH-23390 (0 or 1 mg/L bath concentration), a D1-R antagonist, and subsequently exposed to ethanol (0, 0.25, 0.5, 1.0 % v/v). To explore potential underlying mechanisms, we quantified levels of dopamine, DOPAC, serotonin, and 5-HIAA from whole-brain tissue using high-precision liquid chromatography.

RESULTS

We found pre-treatment with the D1-R antagonist to attenuate locomotor activity independent of ethanol concentration. Furthermore, unlike ethanol, D1-R antagonism did not alter behavioral responses associated with fear and anxiety. Pre-treatment with SCH-23390 decreased levels of dopamine and DOPAC, but this effect was also independent of ethanol concentration. The D1-R antagonist also reduced serotonin and 5-hydroxyindole acetic acid (5-HIAA) levels.

CONCLUSION

These results suggest a multifaceted and at least partially independent role of dopamine D1 receptors in ethanol-induced locomotor activity and anxiety-related responses as well as in the functioning of the dopaminergic and serotoninergic neurotransmitter systems in zebrafish.

Reduced dopamine release in the nucleus accumbens core of adult rats following adolescent binge alcohol exposure: age and dose-dependent analysis

RATIONALE

Alcohol use in adolescence is one of the most significant predictors of alcohol dependence in adulthood, yet the neurochemical mechanisms underlying this heightened vulnerability remain unknown. Whereas research has focused on characterizing adaptations in the mesolimbic dopamine (DA) system following ethanol exposure in adolescence, whether these changes persist into adulthood has yet to be determined.

OBJECTIVES

The objective of this study is to investigate the effects of binge-intermittent ethanol administration in adolescence (P30-50) or early adulthood (P60-80) on DA in the nucleus accumbens (NAc) core after an ethanol challenge in adulthood following a period of abstinence.

METHODS

Male Sprague Dawley rats (n = 160) were administered intermittent ethanol injections, 1 or 3 g/kg, intraperitoneally (i.p.) every other day for 20 days starting on either P30 or 60. Following an ethanol-free period of either 7, 14, or 28 days, we measured DA efflux following an ethanol challenge (3 g/kg, i.p.) using electrochemical recording electrodes bilaterally implanted into the NAc core.

RESULTS

Moderate-dose ethanol administration (1 g/kg, i.p.) during adolescence significantly decreased ethanol-evoked DA release in adulthood at 7 and 14 days, but not 28 days, following pretreatment exposure compared to saline controls. Relative to rats pretreated with ethanol in adulthood, moderate-dose ethanol in adolescence significantly reduced DA efflux at all time points measured. Additionally, adult rats pretreated with high dose ethanol administration (3 g/kg, i.p.) displayed significantly decreased DA compared to adolescents after 28 days of withdrawal.

CONCLUSIONS

Binge-intermittent ethanol administration during adolescence may induce age-dependent neuroadaptations in the mesolimbic DA system compared to ethanol-treated adults during protracted ethanol withdrawal.

Greater ethanol inhibition of presynaptic dopamine release in C57BL/6J than DBA/2J mice: Role of nicotinic acetylcholine receptors

The mesolimbic dopamine system, originating in the ventral tegmental area (VTA) and projecting to the nucleus accumbens (NAc), has been heavily implicated in the reinforcing effects of ethanol. Recent slice voltammetry studies have shown that ethanol inhibits dopamine release selectively during high-frequency activity that elicits phasic dopamine release shown to be important for learning and reinforcement. Presently, we examined ethanol inhibition of electrically evoked NAc dopamine in two mouse strains with divergent dopamine responses to ethanol, C57BL/6 (C57) and DBA/2J (DBA) mice. Previous electrophysiology and microdialysis studies have demonstrated greater ethanol-induced VTA dopaminergic firing and NAc dopamine elevations in DBA compared to C57 mice. Additionally, DBA mice have greater ethanol responses in dopamine-related behaviors, including hyperlocomotion and conditioned place preference. Currently, we demonstrate greater sensitivity of ethanol inhibition of NAc dopamine signaling in C57 compared to DBA mice. The reduced sensitivity to ethanol inhibition in DBA mice may contribute to the overall greater ethanol-induced dopamine signaling and related behaviors observed in this strain. NAc cholinergic activity is known to potently modulate terminal dopamine release. Additionally, ethanol is known to interact with multiple aspects of nicotinic acetylcholine receptor activity. Therefore, we examined ethanol-mediated inhibition of dopamine release at two ethanol concentrations (80 and 160 mM) during bath application of the non-selective nicotinic receptor antagonist mecamylamine, as well as compounds selective for the β2-(dihydro-β-erythroidine hydrobromide; DhβE) and α6-(α-conotoxin MII [H9A; L15A]) subunit-containing receptors. Mecamylamine and DhβE decreased dopamine release and reduced ethanol's inhibitory effects on dopamine in both DBA and C57 mice. Further, α-conotoxin also reduced the dopamine release and the dopamine-inhibiting effects of ethanol at the 80 mM, but not 160 mM, concentration. These data suggest that ethanol is acting in part through nicotinic acetylcholine receptors, or downstream effectors, to reduce dopamine release during high-frequency activity.

Disinhibition of olfaction: human olfactory performance improves following low levels of alcohol

We hypothesize that true human olfactory abilities are obscured by cortical inhibition. Alcohol reduces inhibition. We therefore tested the hypothesis that olfactory abilities will improve following alcohol consumption. We measured olfaction in 85 subjects, 45 in a between-subjects design, and 40 in a repeated-measures within-subjects design before and after consumption of alcoholic or non-alcoholic beverages. Subjects were also assessed using neurocognitive measures of inhibition. Following alcohol consumption, blood alcohol levels ranged from 0.005% to 0.11%. Across subjects, before any consumption of alcohol, we found that individuals who were less inhibited had lower (better) olfactory detection thresholds (r=0.68, p<0.005). Moreover, after alcohol consumption, subjects with low alcohol levels could make olfactory discriminations that subjects with 0% alcohol could not make (chance=33%, alcohol=51.3±22.7%, control=34.7±31.6%, t(43)=2.03, p<0.05). Within subjects, we found correlations between levels of alcohol and olfactory detection (r=0.63, p<0.005) and discrimination (r=-0.50, p<0.05), such that performance was improved at low levels of alcohol (significantly better than baseline for detection) and deteriorated at higher levels of alcohol. Finally, levels of alcohol-induced improved olfactory discrimination were correlated with levels of alcohol-induced cognitive disinhibition (r=0.48, p<0.05). Although we cannot rule out alternative non-inhibitory alcohol-induced routes of influence, we conclude that improved olfaction at low levels of alcohol supports the notion of an inhibitory mechanism obscuring true olfactory abilities.

History of childhood adversity is positively associated with ventral striatal dopamine responses to amphetamine

RATIONALE

Childhood exposure to severe or chronic trauma is an important risk factor for the later development of adult mental health problems, such as substance abuse. Even in nonclinical samples of healthy adults, persons with a history of significant childhood adversity seem to experience greater psychological distress than those without this history. Evidence from rodent studies suggests that early life stress may impair dopamine function in ways that increase risks for drug abuse. However, the degree to which these findings translate to other species remains unclear.

OBJECTIVES

This study was conducted to examine associations between childhood adversity and dopamine and subjective responses to amphetamine in humans.

METHODS

Following intake assessment, 28 healthy male and female adults, aged 18-29 years, underwent two consecutive 90-min positron emission tomography studies with high specific activity [(11)C]raclopride. The first scan was preceded by intravenous saline; the second by amphetamine (AMPH 0.3 mg/kg).

RESULTS

Consistent with prior literature, findings showed positive associations between childhood trauma and current levels of perceived stress. Moreover, greater number of traumatic events and higher levels of perceived stress were each associated with higher ventral striatal dopamine responses to AMPH. Findings of mediation analyses further showed that a portion of the relationship between childhood trauma and dopamine release may be mediated by perceived stress.

CONCLUSIONS

Overall, results are consistent with preclinical findings suggesting that early trauma may lead to enhanced sensitivity to psychostimulants and that this mechanism may underlie increased vulnerability for drug abuse.

Chronic psychosocial stress causes delayed extinction and exacerbates reinstatement of ethanol-induced conditioned place preference in mice

RATIONALE

We have shown previously, using an animal model of voluntary ethanol intake and ethanol-conditioned place preference (EtOH-CPP), that exposure to chronic psychosocial stress induces increased ethanol intake and EtOH-CPP acquisition in mice.

OBJECTIVE

Here, we examined the impact of chronic subordinate colony (CSC) exposure on EtOH-CPP extinction, as well as ethanol-induced reinstatement of CPP.

METHODS

Mice were conditioned with saline or 1.5 g/kg ethanol and were tested in the EtOH-CPP model. In the first experiment, the mice were subjected to 19 days of chronic stress, and EtOH-CPP extinction was assessed during seven daily trials without ethanol injection. In the second experiment and after the EtOH-CPP test, the mice were subjected to 7 days of extinction trials before the 19 days of chronic stress. Drug-induced EtOH-CPP reinstatement was induced by a priming injection of 0.5 g/kg ethanol.

RESULTS

Compared to the single-housed colony mice, CSC mice exhibited increased anxiety-like behavior in the elevated plus maze (EPM) and the open field tests. Interestingly, the CSC mice showed delayed EtOH-CPP extinction. More importantly, CSC mice showed increased alcohol-induced reinstatement of the EtOH-CPP behavior.

CONCLUSION

Taken together, this study indicates that chronic psychosocial stress can have long-term effects on EtOH-CPP extinction as well as drug-induced reinstatement behavior and may provide a suitable model to study the latent effects of chronic psychosocial stress on extinction and relapse to drug abuse.

Frequency-dependent effects of ethanol on dopamine release in the nucleus accumbens

BACKGROUND

Ethanol (EtOH) is known to have excitatory effects on dopamine (DA) release, with moderate-to-high doses (0.5 to 2.5 g/kg) of acute EtOH enhancing DA neuron firing rates in the ventral tegmental area (VTA) and DA levels in the nucleus accumbens (NAc). EtOH has also been shown to reduce DA activity, with moderate doses (1 to 2 g/kg) attenuating electrically evoked release, and higher doses (5 g/kg) decreasing NAc DA levels, demonstrating a biphasic effect of EtOH on DA release. The purpose of the current study was to evaluate EtOH's inhibitory effects on NAc DA terminal release under low- and high-frequency stimulation conditions.

METHODS

Using fast-scan cyclic voltammetry in NAc slices from C57BL/6J mice, we examined EtOH's (40 to 160 mM) effects on DA release under several different stimulation parameters, varying frequency (5 to 125 Hz), number of pulses (1 to 10), and stimulation intensity (50 to 350 μA). Additionally, calcium concentrations were manipulated under high-frequency stimulation conditions (20 Hz, 10 pulses, 350 μA) to determine whether EtOH's effects were dependent upon calcium concentration, and by extension, the amount of DA release.

RESULTS

Acute EtOH (40 to 160 mM) inhibited DA release to a greater extent under high-frequency, multiple-pulse stimulation conditions, with increased sensitivity at 5 and 10 pulses and frequencies of 20 Hz or higher. High-frequency, multiple-pulse stimulations also resulted in greater DA release compared with single-pulse release, which was controlled by reducing stimulation intensity. Under reduced DA conditions, high-frequency stimulations still showed increased EtOH sensitivity. Reducing calcium levels also decreased DA release at high-frequency stimulations, but did not affect EtOH sensitivity.

CONCLUSIONS

EtOH appears to inhibit DA release at NAc terminals under high-frequency stimulation conditions that are similar to release events observed during phasic burst firing in DAergic neurons, suggesting that EtOH may provide inhibition of DA terminals selectively during phasic signaling, while leaving tonic DA terminal activity unaffected.

Synaptic effects induced by alcohol

Ethanol (EtOH) has effects on numerous cellular molecular targets, and alterations in synaptic function are prominent among these effects. Acute exposure to EtOH activates or inhibits the function of proteins involved in synaptic transmission, while chronic exposure often produces opposing and/or compensatory/homeostatic effects on the expression, localization, and function of these proteins. Interactions between different neurotransmitters (e.g., neuropeptide effects on release of small molecule transmitters) can also influence both acute and chronic EtOH actions. Studies in intact animals indicate that the proteins affected by EtOH also play roles in the neural actions of the drug, including acute intoxication, tolerance, dependence, and the seeking and drinking of EtOH. This chapter reviews the literature describing these acute and chronic synaptic effects of EtOH and their relevance for synaptic transmission, plasticity, and behavior.

Amphetamine modifies ethanol intake of psychosocially stressed male rats

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

Stress and the HPA axis: role of glucocorticoids in alcohol dependence

Stress has long been suggested to be an important correlate of uncontrolled drinking and relapse. An important hormonal response system to stress-the hypothalamic-pituitary-adrenal (HPA) axis-may be involved in this process, particularly stress hormones known as glucocorticoids and primarily cortisol. The actions of this hormone system normally are tightly regulated to ensure that the body can respond quickly to stressful events and return to a normal state just as rapidly. The main determinants of HPA axis activity are genetic background, early-life environment, and current life stress. Alterations in HPA axis regulation are associated with problematic alcohol use and dependence; however, the nature of this dysregulation appears to vary with respect to stage of alcohol dependence. Much of this research has focused specifically on the role of cortisol in the risk for, development of, and relapse to chronic alcohol use. These studies found that cortisol can interact with the brain's reward system, which may contribute to alcohol's reinforcing effects. Cortisol also can influence a person's cognitive processes, promoting habit-based learning, which may contribute to habit formation and risk of relapse. Finally, cortisol levels during abstinence may be useful clinical indicators of relapse vulnerability in alcohol-dependent people.

Dopamine dynamics associated with, and resulting from, schedule-induced alcohol self-administration: analyses in dopamine transporter knockout mice

Preclinical and clinical evidence suggest an association between alcoholism and the primary regulator of extracellular dopamine concentrations, the dopamine transporter (DAT). However, the nature of this association is unclear. We determined if 10 days of voluntary alcohol self-administration followed by withdrawal could directly alter DAT function, or if genetically mediated changes in DAT function and/or availability could influence vulnerability to alcohol abuse. Heterozygous (DAT+/-) and homozygous mutant (DAT-/-) and wild-type (DAT+/+) mice were allowed to consume 5% alcohol in a schedule-induced polydipsia (SIP) task. In vivo fixed potential amperometry in anesthetized mice was used to (1) identify functional characteristics of mesoaccumbens dopamine neurons related to genotype, including dopamine autoreceptor (DAR) sensitivity, DAT efficiency, and DAT capacity, (2) determine if any of these characteristics correlated with alcohol drinking observed in DAT+/+ and DAT+/- animals, and (3) determine if SIP-alcohol self-administration altered DAR sensitivity, DAT efficiency, and DAT capacity by comparing these characteristics in wild-type (DAT+/+) mice that were SIP-alcohol naïve, with those that had undergone SIP-alcohol testing. DAT-/- mice consumed significantly less alcohol during testing and this behavioral difference was related to significant differences in DAR sensitivity, DAT efficiency, and DAT capacity. These functional characteristics were correlated to varying degrees with g/kg alcohol consumption in DAT+/+ and DAT+/- mice. DAR sensitivity was consistently reduced and DAT efficiency was enhanced in SIP-alcohol-experienced DAT+/+ mice when compared with naïve animals. These results indicate that DAR sensitivity is reduced by SIP-alcohol consumption and that DAT efficiency is modified by genotype and SIP-alcohol exposure. DAT capacity appeared to be strictly associated with SIP-alcohol consumption.

Differential effects of stress on [¹¹C]raclopride and [¹¹C]MNPA binding to striatal D₂/D₃ dopamine receptors: a PET study in conscious monkeys

It has been reported that stress and facilitation of dopamine neuronal system are closely related. In the present study, the effects of stress on the binding of antagonist-based [¹¹C]raclopride and agonist-based (R)-2-CH3O-N-n- propylnorapomorphine ([¹¹C]MNPA) to D₂/D₃ receptors were evaluated in the striatum of conscious monkey brain. The stress state assessed from plasma cortisol level was negatively correlated with [¹¹C]raclopride binding as expected. It was noteworthy that [¹¹C]MNPA binding exhibited a positive correlation with stress state; thus, the animals with higher cortisol levels showed higher binding to D₂/D₃ receptors.

Mindfulness-based treatments for co-occurring depression and substance use disorders: what can we learn from the brain?

Both depression and substance use disorders represent major global public health concerns and are often co-occurring. Although there are ongoing discoveries regarding the pathophysiology and treatment of each condition, common mechanisms and effective treatments for co-occurring depression and substance abuse remain elusive. Mindfulness training has been shown recently to benefit both depression and substance use disorders, suggesting that this approach may target common behavioral and neurobiological processes. However, it remains unclear whether these pathways constitute specific shared neurobiological mechanisms or more extensive components universal to the broader human experience of psychological distress or suffering.We offer a theoretical, clinical and neurobiological perspective of the overlaps between these disorders, highlight common neural pathways that play a role in depression and substance use disorders and discuss how these commonalities may frame our conceptualization and treatment of co-occurring disorders. Finally, we discuss how advances in our understanding of potential mechanisms of mindfulness training may offer not only unique effects on depression and substance use, but also offer promise for treatment of co-occurring disorders.

Stimulated dopamine overflow and alpha-synuclein expression in the nucleus accumbens core distinguish rats bred for differential ethanol preference

The key neurochemical systems and structures involved in the predisposition to substance abuse and preference to ethanol (EtOH) are not known in detail but clearly dopamine (DA) is an important modulator of addiction. Recent data indicate that alpha-synuclein (alpha-syn), a pre-synaptic protein, plays a role in regulation of DA release from the pre-synaptic terminals in striatum and the expression of this protein is different after drug abuse or following abstinence. In the present work, we analysed stimulated DA overflow in the dorsal and ventral striatum in EtOH naïve alko alchohol (AA) and alko non-alchohol (ANA) rats selected for more than 100 generations for their differential EtOH preference. In the same structures, we studied the expression of alpha-syn using western blotting. AA rats, in comparison with ANA rats, showed a marked reduction of stimulated peak DA overflow and higher levels of alpha-syn in the nucleus accumbens core. In the same structure, DA re-uptake was increased in AA rats in comparison with ANA rats. The effects of EtOH at low (0.1 g/kg) and higher (3 mg/kg) doses on DA overflow measured in the nucleus accumbens shell were similar in both lines. These results indicate that high expression of alpha-syn may contribute to the reduced DA overflow and the possible activation of re-uptake in the nucleus accumbens core of AA rats in comparison with ANA rats.

Association of amphetamine-induced striatal dopamine release and cortisol responses to psychological stress

Preclinical studies have shown that stress and glucocorticoids increase mesolimbic dopamine (DA) and thereby facilitate psychostimulant self-administration. The relationship between stress-induced cortisol and mesolimbic DA responses to psychostimulants has not been studied in humans. To test the hypotheses that glucocorticoid responses to psychological stress are correlated with DA and subjective responses to psychostimulants in humans, 25 healthy adults (18-29 years) completed the Trier Social Stress Test (TSST) and two positron emission tomography (PET) scans with high-specific [11C]raclopride. The first scan was preceded by intravenous saline and the second by amphetamine (AMPH). Findings showed that stress-induced cortisol levels were positively associated with AMPH-induced DA release in the ventral striatum and other striatal regions. Subjects with higher cortisol responses to stress also reported more positive subjective drug effects with AMPH than subjects with lower responses. The results are consistent with preclinical findings showing an interrelationship between glucocorticoids and mesolimbic DA dynamics, which may influence psychostimulant self-administration in humans.

Effects of chronic alcohol exposure on dopamine uptake in rat nucleus accumbens and caudate putamen

RATIONALE

Existing data strongly suggest that alcohol affects dopamine (DA) neurotransmission in the brain. However, many questions remain about the effects of alcohol on the delicate equilibrium between such neurochemical processes as DA release and uptake. Dysregulation of these processes in the mesolimbic and nigrostriatal systems after chronic alcohol ingestion could be a neuroadaptation contributing to dependence.

OBJECTIVES

In the present study, we have employed an alcohol vapor inhalation model to characterize the effects of chronic alcohol exposure on DA dynamics in rat nucleus accumbens (NAc) and caudate putamen (CP) using fast-scan cyclic voltammetry (FSCV) in brain slices. This method provides a unique view of real-time, spatially resolved changes in DA concentration.

RESULTS

We found that chronic alcohol exposure enhanced DA uptake rates in rat NAc and CP. These changes would have the effect of down-regulating extracellular DA levels, presumably a compensatory effect related to increased DA release by repeated alcohol exposure. The sensitivity of terminal release-regulating DA autoreceptors was not different in alcohol-exposed rats compared with alcohol-naïve animals.

CONCLUSIONS

The DA uptake changes after chronic alcohol exposure documented here using FSCV may be associated with a compensatory response of the DA system aimed at decreasing DA signaling. Alterations in autoreceptor function may require relatively long lasting alcohol exposure.

Impulsivity and chronic stress are associated with amphetamine-induced striatal dopamine release

A challenging question that continues to plague the field of addiction is why some individuals are more vulnerable for substance use disorders than others. Several important risk factors for substance abuse have been identified in clinical studies, including trait impulsivity and environmental stress. However, the neurobiological mechanisms that underlie the relationships remain poorly understood. The purpose of this study was to examine associations among impulsivity, stress, and striatal dopamine (DA) responses to amphetamine (AMPH) in humans. Forty healthy M, F adults, ages 18-29 years, completed self-report measures of trait impulsivity, life events stress, and perceived stress. Subjects subsequently underwent two consecutive 90-min positron emission tomography (PET) studies with high specific activity [11C]raclopride. The first scan was preceded by an intravenous injection of saline; the second was preceded by 0.3 mg/kg AMPH. Findings showed that high impulsivity was associated with blunted right ventral striatal DA release. However, effects were modified by a significant interaction with life events stress. Dopamine release was greater in low vs. high impulsivity subjects under conditions of low or moderate stress. Under conditions of high stress, both groups had low DA release. Subjects with high impulsivity reported more pleasant effects with AMPH than subjects with low impulsivity. In contrast, stress was negatively associated with pleasant drug effects. No associations were observed between impulsivity or stress and cortisol responses to AMPH. The findings are consistent with notions that blunted DA responses represent an endophenotype for substance use disorders.

No role of the dopamine transporter in acute ethanol effects on striatal dopamine dynamics

The acute effects of ethanol on dopamine (DA) release and clearance in the caudate-putamen were evaluated in wild-type and dopamine transporter (DAT) knockout (DAT-KO) mice, using microdialysis and voltammetry. Dialysate DA levels were elevated, approximately 80% above baseline levels, after administration of 2 g/kg ethanol in both wild-type and DAT-KO mice. In brain slices containing the caudate-putamen, a low (20 mM) concentration of ethanol produced no change in electrically stimulated DA release in either wild-type or DAT-KO mice. A high concentration (200 mM) of ethanol caused a similar decrease in DA release in slices from both types of mice. DA clearance was unaltered across the genotypes at low and high concentrations of ethanol. The fact that ethanol had similar effects in wild-type and DAT-KO mice, measured by in vivo microdialysis or brain slice voltammetry, supports the idea that acute ethanol does not interact with the DAT to produce its effects on the DA system.

Effect of moderate ethanol dose on dopamine uptake in rat nucleus accumbens in vivo

The present study was designed to evaluate the effects of a moderate dose of ethanol (1 g/kg) on dopamine (DA) dynamics in rat nucleus accumbens (NAc) using fast-scan cyclic voltammetry. Voltammetric recordings were made every 100 ms at a carbon fiber microelectrode, positioned in the NAc core. Acute ethanol did not significantly alter DA uptake parameters (K(m) and V(max)), but amplitudes of the DA signals were decreased after the drug in both freely moving and anesthetized rats. Therefore, the present in vivo voltammetry results suggest that DA uptake changes are not involved in ethanol-induced increases in extracellular DA concentrations.

Local effects of acute ethanol on dopamine neurotransmission in the ventral striatum in C57BL/6 mice

In this study, fast-scan cyclic voltammetry in brain slices was used to evaluate the effects of acute ethanol on dopamine terminal release and uptake in the nucleus accumbens of C57BL/6 mice. We found that pharmacologically relevant concentrations of ethanol (20 and 100 mM) did not alter electrically evoked dopamine release, while the highest concentration (200 mM) significantly decreased release (approximately 45%). No significant changes were observed in the rate of dopamine uptake after ethanol (20, 100 or 200 mM). In addition, it was established that a moderate dose (2 g/kg, i.p.) of ethanol did not alter the rate of dopamine synthesis, measured as L-dihydroxyphenylalanine (L-DOPA) accumulation. However, a high dose (5 g/kg, i.p.) of ethanol significantly increased the levels of L-DOPA to 60% above the control value. These data are consistent with earlier findings obtained in brain slices from rats; dopamine release, but not clearance, is affected by acute ethanol.

Acute Ethanol Decreases Dopamine Transporter Velocity in Rat Striatum: In Vivo and In Vitro Electrochemical Measurements

BACKGROUND

Ethanol increases dopamine transporter (DAT) velocity when measured in cell expression systems, but its effects in vivo are mixed. The present experiments examined the effect of acute ethanol on dopamine transmission, particularly DAT velocity, in anesthetized animals as well as rat striatal suspensions.

METHODS

To determine the effect of acute ethanol on DAT function in vivo, we measured dopamine uptake in real time using fast-scan cyclic voltammetry and constant potential amperometry in the olfactory tubercle of anesthetized rats. Dopamine fibers were electrically stimulated, and the resulting transient dopamine signals were analyzed to describe the release and uptake kinetics. We also measured the effect of ethanol on DAT velocity in vitro in striatal tissue suspensions using rotating disk electrode voltammetry.

RESULTS

Ethanol (2.5 and 4 g/kg, intraperitoneally) decreased the electrically stimulated dopamine signal in the olfactory tubercle by 35-55%. The slope of the clearance curve of dopamine was 40% shallower after both doses of ethanol, indicating slower uptake. Modeling the data using Michaelis-Menten uptake kinetics showed that the slower uptake was due to a decrease in DAT V(max). These results were confirmed in vitro, because ethanol decreased the velocity of dopamine uptake by 35% in striatal tissue suspensions.

CONCLUSIONS

These results indicate that acute ethanol decreases DAT function in rat dorsal and ventral striatum in anesthetized rats and tissue suspensions, in contrast to its effects on human DAT expressed in single cells. Given the variety of molecular targets of ethanol in the brain, including the DAT itself, it is likely that several mechanisms converge to produce a net effect on DAT regulation and function that could very well be different in intact tissue versus single cells.

Relationships among ventral striatal dopamine release, cortisol secretion, and subjective responses to amphetamine

There is evidence that stress and glucocorticoids alter drug self-administration and mesolimbic dopamine (DA) activity in preclinical models. The primary purpose of this study was to test the hypothesis that glucocorticoids are associated with psychostimulant reinforcement and DA release in humans. In total, 16 healthy adults, ages 18-27 years, underwent two consecutive 90-min PET studies with high specific activity [11C]raclopride. The first scan was preceded by intravenous saline, and the second by intravenous amphetamine (AMPH 0.3 mg/kg). DA release was defined as the percent change in raclopride binding between the placebo and AMPH scans. Measures of subjective drug effects, plasma cortisol, and growth hormone (GH) were obtained. Findings showed that cortisol levels were positively associated with AMPH-induced DA release in the left ventral striatum (LVS) and the dorsal putamen. Subjects with higher cortisol responses to AMPH also reported more positive subjective drug effects than subjects with lower cortisol responses; no association was observed between cortisol levels and negative drug effects. Higher ratings of positive drug effects were also associated with greater DA release in the LVS, dorsal putamen, and dorsal caudate. A general lack of relationship was observed between GH responses to AMPH and DA release or subjective drug responses. Our findings provide evidence of interrelationships between glucocorticoid levels, subjective responses to IV AMPH, and brain DA release in humans. The results are consistent with those of preclinical studies, suggesting that individual differences in HPA axis function may influence vulnerability to alcohol and drug dependence in humans.

Low doses of alcohol have a selective effect on the recognition of happy facial expressions

Alcohol is one of the most widely used recreational drugs, yet it is associated with undesirable social behaviour. It is used primarily for its psychoactive properties, increasing sociability and talkativeness. We hypothesize that low doses of alcohol can improve the performance related to positive emotional cognition. In this experiment, we examined the effect of low doses of alcohol on the processing of emotional facial expressions. Fifteen young male volunteers drank alcohol at volumes of 30, 60, 120 ml (0.14, 0.28, 0.56 g/kg) and performed discrimination tasks on morphed facial emotion expressions of anger, happiness, sadness and surprise-neutral. One-way ANOVA co-varying pretreatment performances revealed significant differences between alcohol levels in happy face discrimination ( p<0.01). Bonferroni correction demonstrated that low doses of alcohol caused a significantly better discrimination of happy faces, and that the performances were worse with higher doses ( p<0.001). No significance was observed with the other three emotional faces. These results indicate that low doses of alcohol affect positive emotional cognition of happy facial expressions.

Effects of chronic alcohol ingestion on the mesostriatal dopamine system in the rat

The effects of chronic alcohol use on the mesostriatal dopamine (DA) system remain relatively unknown. The aim of the present study was to assess multiple measures of the status of the mesostriatal DA system in rats chronically fed an alcohol diet for approximately 1 year. Tissue levels of DA and its metabolite, 3,4-dihydroxyphenylacetic acid, were significantly decreased in both the dorsal striatum (34 and 33%, respectively) and ventral striatum (33 and 36%, respectively) in alcohol-fed rats compared to pair-fed matched controls. Western blotting revealed a mean 20% decrease in tyrosine hydroxylase protein levels in the dorsal and ventral striatum of alcohol-fed animals while dopamine transporter protein levels from the same animals were significantly increased compared to controls (mean 60% increase for the dorsal and ventral striatum). The present results demonstrate significant alterations in the mesostriatal DA system after 1 year of chronic alcohol use. It is possible that the observed changes in DA synthesis and re-uptake measures result in altered intracellular and extracellular DA levels, perhaps contributing to the addictive properties of alcohol.