Dopamine and Alcohol Dependence: From Bench to Clinic

As part of a collaborative effort examining the effects of long-term alcohol self-administration in rhesus macaques, we examined DS dopamine signaling using fast-scan cyclic voltammetry. We found that chronic alcohol self-administration resulted in several dopamine system adaptations. Following long-term alcohol consumption, male macaques, regardless of abstinence status, had reduced dopamine release in putamen, while only male macaques in abstinence had reduced dopamine release in caudate. In contrast, female macaques had enhanced dopamine release in the caudate, but not putamen. Dopamine uptake was also enhanced in females, but not males (regardless of abstinence state). We also found that dopamine D2/3 autoreceptor function was reduced in male, but not female, alcohol drinkers relative to control groups.

As mentioned previously, in addition the affecting the dopamine system directly, alcohol interacts with the mesolimbic dopamine system indirectly via several other neurotransmitters. There is a wide range of such compounds, and here, we will only mention a few, specifically targeting glycine receptors and nAChRs, with a clear interaction with dopamine transmission in the mesolimbic dopamine system [64]. Underlying the brain changes and neuroadaptations are the reward and stress circuits of the brain. A neural circuit comprises of a series of neurons which send electro chemical signals to one another. An activated neuron sends chemical signaling molecules called neurotransmitters through the neural circuit which bind to specific molecules called the receptors.

The dopamine system and brain reward circuitry

Acutely, in vivo alcohol administration dose-dependently increases cortical, mesolimbic, and nigrostriatal dopamine in rodents [36]; an effect attributed to enhanced dopamine neuron firing [37]. However, in rodent and macaque brain slices, an acute alcohol challenge following chronic alcohol exposure (inhalation or drinking) decreases dopamine release in the nucleus accumbens (NAc) in vivo and ex vivo preparations [24, 38]. mirtazapine with alcohol Beyond the NAc, chronic alcohol exposure has varied effects on dopamine release that are brain region and species dependent.

Future experiments will need to assess the relationship between the changes in dopaminergic transmission and other striatal excitability and synaptic alterations following chronic alcohol exposure and intake. While this may be difficult to do in NHPs, where experimental manipulations are limited, parallel experiments in rodent models may be able to provide useful information. For example, we know that GABAergic transmission in striatum is altered in a similar fashion after chronic alcohol exposure in mice and monkeys, and similar effects on dopamine release are observed in some strains of mice and monkeys. Thus, the connection between the trans-species conserved changes can be explored in the more tractable rodent models. In line with the hypothesis that a partial dopamine D2 agonist would block the reinforcing effects of alcohol, aripiprazole attenuates alcohol’s ability to increase the locomotor activity in mice [178, 179](an indirect measure of activation of the mesolimbic dopamine system).

Dopamine and Alcohol Dependence: From Bench to Clinic

1. Dopamine plays many important roles in the body, affecting moods, memory and sensations of pleasure and pain.
2. Dopamine also activates memory circuits in other parts of the brain that remember this pleasant experience and leave you thirsting for more.
3. Dopamine is a neuromodulator that is used by neurons in several brain regions involved in motivation and reinforcement, most importantly the nucleus accumbens (NAc).
4. Although there exists promising preclinical results, the majority of placebo‐controlled randomized clinical trials with traditional dopamine antagonists and agonists have so far have been discouraging.

It should also be noted that our study is the first to examine long-term alcohol effects on dopamine release in the putamen of NHPs and to demonstrate that acetylcholine driven dopamine release is conserved across rodent and NHP species. At low doses, bromocriptine can reduce alcohol consumption in animals [171]; it is possible that low‐dose dopamine agonists preferentially augment autoreceptor function, thereby decreasing dopamine turnover and blunting the rewarding effects of alcohol. An early double‐blinded study [172] reported that bromocriptine reduced alcohol craving in alcohol‐dependent patients with a specific genotype of the dopamine D2 receptor gene (i.e. the A1/A1 and A1/A2 genotypes). However, subsequent double‐blind placebo‐controlled trials found no effect on relapse or related behaviours [173, 174]. Currently, due to the knowledge of the addictive potential of dopamine agonists, combined with the lack of consistent findings from clinical studies, it is suggested that dopamine receptor agonists do not hold promise as a treatment for alcohol dependence.

The mesocorticolimbic dopamine system has an established role in driving the rewarding sensations from natural rewards such as food, sex and exercise, which are important behaviours to ensure our survival [6, 7] as well as among drugs of abuse, including alcohol (for review see [8]). The physiological importance of the mesocorticolimbic dopamine system is highlighted by its evolutionary stability and conservation in primitive invertebrates, such as, flatworms, all the way up to primates, including humans. It was identified serendipitously in the 1950s when Olds and Milner found that rats self‐administer electrical currents into certain specific brain regions [9]. These findings were later corroborated by studies showing that rats favoured electrical stimulation in the same specific brain regions, over natural rewards [10]. The primary neurotransmitter regulating the rewarding sensation was determined to be dopamine [11].

While drinking initially boosts a person’s dopamine levels, the brain adapts to the dopamine overload with continued alcohol use. It produces less of the neurotransmitter, reducing the number of dopamine receptors in the body and increasing dopamine transporters, which carry away the excess dopamine. Researchers are investigating whether drugs that normalize dopamine levels in the brain might be effective in reducing alcohol cravings and treating alcoholism. The atypical antipsychotic tiapride has been found to be efficacious in reducing alcohol drinking two placebo‐controlled clinical trials [158, 159]. A small study in twenty alcohol‐dependent individuals, with significant levels of anxiety or depression, showed that tiapride treatment causes a reduced alcohol intake as well as prolonged periods of abstinence [158]. In the largest of the studies [159], 100 recently abstinent alcohol‐dependent patients were randomized to 300 mg of tiapride or placebo for a 3‐month treatment period.

What the study said

The contrasting microdialysis results in alcohol‐drinking alpha-pyrrolidinopentiophenone function versus alcohol‐naïve rats highlight OSU6162´s ability to modulate the dopamine output dependent on the prevailing dopaminergic tone. Furthermore, these results indicate that OSU6162 might have the ability to attenuate alcohol‐mediated behaviours by counteracting the hypo‐dopaminergic state induced by long‐term drinking. Collectively, together with the finding that OSU6162 did not induce conditioned place preference [29] (an indication that the compound has no rewarding properties of its own), these results indicate that OSU6162 has many of the favourable characteristics of a potential medication for alcohol dependence. For the determination of dopamine transient uptake kinetics, the modeling module in DEMON was used as previously described [30].

The gene encoding GABRA1 is located on chromosome 5 at 5q34-35 while the gene encoding GABRA6 is located on the same chromosome at 5q34. According to a study by,[62] a significant correlation was found with the GABRA1 genotype and Collaborative Study of the Genetics of Alcoholism (COGA) AD, history of blackouts, age at first drunkenness as well as the level of response to alcohol. The study concludes by stating that the efforts to characterize genetic contributions to AD may benefit by examining alcohol-related behaviors in addition to clinical AD. Dopaminergic neurons that relay information to the NAc shell are extremely sensitive to alcohol. For example, in studies performed in rats, alcohol injected into the blood in amounts as low as 2 to 4 milligrams per kilogram of body weight increased dopamine release in the NAc shell and maintained chronic alcohol self-administration (Lyness and Smith 1992). In rats, oral alcohol uptake also stimulates dopamine release in the NAc (Weiss et al. 1995).

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Nonetheless, it is interesting to note that the previously reported drinking data from Cohort 3 rhesus macaques showed an alcohol deprivation effect-like phenomenon in which subjects robustly increased their ethanol consumption for 1 month following each abstinence period [32]. Furthermore, the trend toward decreased dopamine release in the males with no abstinence might have become significant had those subjects been put through abstinence periods like the male subjects in Cohort 3 of this study. A series of experiments in outbred rats show that the dopamine stabilizer OSU6162 attenuates several alcohol‐mediated behaviours including voluntary alcohol intake, alcohol withdrawal symptoms and cue/priming‐induced reinstatement of alcohol seeking in long‐term drinking rats [196]. Furthermore, OSU6162 blunted alcohol‐induced dopamine output in the NAc of alcohol‐naïve rats [196], indicating that OSU6162 has the ability to attenuate the rewarding effects of alcohol. In contrast, a more recent microdialysis study conducted in long‐term drinking rats, showed that OSU6162, compared to vehicle‐pretreatment, had no significant effect on the alcohol‐induced dopamine peak [29].

Other research indicates that some people tend to have a higher release of and response to dopamine than gifts to celebrate sobriety others. In addition, those individuals may be predisposed to drink more heavily and develop an alcohol addiction. A small study by researchers at Columbia University revealed that the dopamine produced during drinking is concentrated in the brain’s reward center. The study further found that men exhibit a greater release of dopamine when they drink than women. As a result, people with an alcohol addiction may consume even more alcohol in an unconscious effort to boost their dopamine levels and get that spark back. Dopamine also activates memory circuits in other parts of the brain that remember this pleasant experience and leave you thirsting for more.

Briefly, the dopamine affinity for the transporter (Km; set to 0.16 µM) was held constant and the dopamine peak height was determined empirically for each file and used for determination of Vmax (dopamine uptake rate), which was altered to best fit the empirically obtained dopamine transients. To examine D2/3 dopamine autoreceptor function, the D2/3 dopamine receptor agonist, quinpirole (30 nM), was bath applied for 30 min and was followed by application of the D2-like dopamine receptor antagonist sulpiride (2 µM) for 15 min. To examine differences between tonic and phasic release, we applied stimuli at varying frequencies before and after the application of the β2 subunit-containing nAChR antagonist, dihydro-β-erythroidine hydrobromide (DHβE; 1 µM).

We found that long-term alcohol consumption altered dorsal striatal dopamine release and uptake in a sex- and subregion-dependent manner. We further found that regulation of dopamine release by D2/3 dopamine autoreceptors was altered by long-term alcohol consumption in male, but not female, rhesus macaques regardless of abstinence status. For example, long-term alcohol self-administration resulted in decreased dopamine uptake rates in the dorsolateral caudate of male cynomolgus macaques [22, 24]. This group also found no difference in the quinpirole-mediated inhibition of dopamine release between alcohol and control male cynomolgus macaques [24]. It is likely that species, striatal subregion, and intake duration (6 months in the previous study versus 1 year in the present study) differences may account for many of the dissimilarities between studies.