Alcohol is a causal factor in more than 200 disease and injury conditions and is attributed to the deaths of over 175,000 people annually in the United States (
1) and 3 million people (5.3% of all deaths) worldwide (
2). The excessive use of alcohol remains highly prevalent, with 10.6% of Americans aged 12 and older (29.5 million) meeting criteria for past-year alcohol use disorder (AUD) (
3). Further, there is a high rate of psychiatric comorbidity in AUD, with depressive disorders being the most common comorbidity (33%–50% [
4]). AUD and depression have a bidirectional relationship, as having one of these disorders increases the risk of the other (
5), and there is some evidence for potential shared vulnerability via genetic predisposition (
6) and brain reward and stress system dysfunction (
7).
Advances in neuroscience research over the past few decades have fostered models of addiction involving a spiraling process associated with increased negative affect and withdrawal symptoms (
8), with alcohol and/or drugs used compulsively to alleviate such aversive states. Related theories suggest alcohol is misused in those prone to negative affect as a form of tension reduction or “self-medication” (
9). The allostasis model of addiction (
10) hypothesizes that excessive alcohol use produces neuroadaptations from an initial positive reinforcement phase to a negative reinforcement phase. Thus, alcohol is consumed primarily to relieve negative affect and withdrawal (i.e., “the dark side of addiction”) rather than for pleasure or reward (
10). Support for these heuristic models is derived from basic science animal models (
7,
8) and human neuroimaging showing decreased reward circuit activity in regular drug users versus nonusers to sexual content (
11) and in heavy versus light drinkers to intravenous alcohol (
12).
Related to experiences of alcohol reward and punishment, alcohol response phenotype predicts the development and maintenance of AUD (
13–
15). These phenotypes vary, but two large longitudinal studies have shown that enhanced sensitivity to the stimulating and rewarding effects of alcohol (
13,
14) and lower sensitivity to its sedating effects (
15) predicts heavy drinking behavior and AUD. Debate remains whether excessive drinking reflects overall acute alcohol tolerance, desire for relief from negative mood states (e.g., depression), or heightened sensitivity to alcohol’s pleasurable effects (
16). Most of this research has examined otherwise healthy young people who drink and are at risk for future alcohol problems, with those meeting criteria for moderate/severe AUD at baseline excluded (
13), so little is known about subjective and motivational alcohol properties in individuals with AUD and comorbid depressive disorders. Examining subjective alcohol effects among these individuals would provide a critical test of theories of alcohol misuse and guide intervention development for these comorbidities.
Thus, to test whether excessive alcohol is associated primarily with relief of negative mood states or reward and enhancement of pleasurable effects, we examined real-time positive and negative subjective effects in individuals with and without AUD and depressive disorders. To this end, we used smartphone-delivered ambulatory monitoring to capture data in participants’ natural environments during an alcohol drinking episode and a comparison non-alcohol drinking episode. The non-alcohol drinking episode allowed us to examine whether participant reports of subjective responses during alcohol use are specific to that substance, versus a nonspecific response bias (
17). This approach used high-resolution ecological momentary assessment (HR-EMA) (
18), a reliable and valid method for measuring alcohol use and subjective effects during excessive drinking events (
17,
19). In this study, we examined a typical drinking event to determine whether persons with AUD and comorbid depressive disorder report a reduction in negative affect, or enhancement of positive affect, as shown previously in nondepressed heavy social (
13,
14) and AUD drinkers (
17,
20). To our knowledge, this represents the first real-time assessment comparing hedonic alcohol effects (stimulation, liking), negative affect, motivational salience (wanting), and sedative effects in individuals with and without AUD and depressive disorders.
Discussion
During alcohol intoxication, people with comorbid AUD and depressive disorder showed a positive alcohol response phenotype characterized by heightened stimulation, liking, and wanting, resembling the response profile of AUD drinkers without depression. Thus, persons with AUD, even those with depressive disorders, experience the pleasurable effects of alcohol soon after consuming alcohol, and these effects remain elevated for much of the course of a heavy drinking bout. In contrast, persons without AUD, regardless of depressive disorder status, engaged in moderate drinking with less pronounced positive-type subjective responses. Finally, alcohol reduced negative affect, but this reduction was small in magnitude and nonspecific to depressive disorder or AUD status. Subjective responses during the alcohol drinking episode differed significantly from those in the non-alcohol drinking episode, validating that participants’ subjective experiences reflected the effects of alcohol rather than a nonspecific response bias (
17). These findings extend prior results of placebo-controlled laboratory studies demonstrating pronounced positive-type alcohol effects in non-comorbid heavy social alcohol users and individuals with AUD (
14,
20).
It is notable that the current findings from natural environment alcohol and non-alcohol drinking episodes do not support the allostasis model’s contention that an initial positive reinforcement phase wanes in addiction, particularly for individuals with conditions characterized by negative affect who are theorized to drink to excess for negative reinforcement factors. As such, this real-time assessment of drinking in comorbid drinkers disputes conventional thinking that the motivation to consume large quantities of alcohol serve as self-medication (
9) to lessen negative affective states (
10) or within a relief drinking profile (
38). Rather, these data indicate that pronounced and prolonged stimulating, hedonic, and motivational rewarding effects of alcohol may underlie alcohol misuse in persons with AUD and comorbid depressive disorder. Finally, the present study findings challenge the widely held concept of global tolerance in addiction (
39). While behavioral tolerance may be present at lower (but not higher) intoxicating alcohol doses in persons with AUD (
40), controlled studies of subjective responses in persons with AUD are lacking. In the present study’s real-time assessment of naturalistic drinking episodes, we found evidence of sensitivity to alcohol’s desirable subjective effects, rather than tolerance to these effects, with persistently high stimulation, hedonic reward, and motivational reward in the alcohol (versus non-alcohol) episodes in persons with AUD, regardless of depression status. While exploratory AUD subgroup analyses yielded small sample sizes, these results indicated that the most pronounced AUD drinkers, i.e., individuals with moderate and severe AUD, exhibited higher alcohol stimulation and hedonic reward compared with those with mild AUD.
Alcohol affects the brain in complex ways (
41), and a greater understanding of the factors that affect vulnerability to AUD and comorbid depression is critical for early identification and more effective prevention and treatment. Despite advances in the measurement of acute alcohol responses in the laboratory and natural environment (
18,
42), only a few studies have examined acute alcohol effects in individuals who drink with co-occurring mental health disorders, with most (
43–
45) but not all (
46), showing higher positive-like alcohol responses in people with psychiatric conditions (bipolar type 1, schizophrenia, anxiety-like disorders). However, none of these studies examined individuals with depressive disorders. Thus, our understanding of alcohol response phenotypes in problem drinkers with affective disturbance is hampered by the paucity of research, lack of inclusion of people with AUD, and small sample sizes (subgroup Ns of 20 to 30). Related, the acute alcohol response findings may have implications for treatment such that for dual diagnosis patients, medications targeting opioidergic, dopaminergic, or impulsivity-related neural mechanisms (
47) to combat hedonic and motivational salience effects of alcohol in combination with antidepressant medications may show more efficacy than medications focused on relief of negative affect or the stress response system (
48). Treatment outcomes could be further optimized with behavioral approaches that address a positive alcohol response phenotype (
49).
While controlled laboratory studies are the gold standard for measuring acute alcohol response phenotypes in persons at risk for AUD (
13,
15) and in the context of the progression and maintenance of AUD (
14,
20), such paradigms have limited external validity. The novel HR-EMA approach used in this study leveraged smartphone technology with brief assessments to bridge prior studies using fixed-dose oral or intravenous alcohol administration (
42) to real-time assessment in the natural environment of persons who drink alcohol. The most notable difference between the laboratory-based controlled dosing paradigm and HR-EMA is that the former allows assessment of biphasic alcohol effects when breath alcohol levels are rising, at its peak, and declining whereas the latter reveals what occurs in the natural environment: steady paced drinking in light or moderate drinkers and progressive excessive drinking to intoxication in risky drinkers (
17). There is also debate about people’s ability to accurately report their internal sensations. Our HR-EMA assessments with psychometrically valid adjective items, a comparison non-alcohol episode, and instructions not requiring participants to ascertain effects based on the stimulus (i.e., beverage consumed) may serve to lessen the potential confounds of complex cognitive appraisals (
50).
The present study featured several strengths, such as enrolling participants with past-year depressive disorders, conducting real-time natural environment assessments to enhance external validity, controlling for eBAC levels in analyses, and demonstrating excellent survey completion rates during the real-world episodes. Also, the participants in the AUD+ group not only represent a clinically relevant sample but also a more excessive drinking group than in most prior studies of heavy social drinkers at risk for AUD. The heavy drinking frequency was 41% and 37% of days for AUD+ participants without and with depressive disorders, respectively, compared with 28% and 22% of days in prior laboratory (
13) and HR-EMA samples (
28). However, there were some limitations. First, to reduce burden, the episodes were limited to 3 hours and so may have missed longer drinking episodes, which may have been particularly important among participants with AUD. Second, while DEP+ participants met past-year criteria for a depressive disorder, those with severe symptoms (e.g., current suicidal ideation) were excluded for safety reasons. Third, alcohol and non-alcohol drinking events were based upon single episodes. However, prior studies have shown 97% agreement between self-report of drinking and alcohol biosensor data (
40), as well as good correspondence of subjective effects across multiple episodes and laboratory sessions (
19). Fourth, while participants were young adults between 21 and 35 years of age, corresponding to the highest rates of heavy drinking across the lifespan (
51), generalizability to older adults may be limited. Of note, the allostasis model does not specify age ranges or duration intervals for the purported stage transitions. Last, as participants completed the mobile assessments outside of the laboratory and in their usual environments, there is the possibility that they misreported their alcohol or other substance use. Alcohol biosensors have the potential to augment future EMA work in this area by providing an objective measure of alcohol use in naturalistic settings (
40,
52,
53).
Positive reinforcement in which alcohol produces hedonic effects may not be solely confined to the early stages of addiction but rather may persist throughout the addiction cycle (
7) and in persons with high negative affect by virtue of depressive disorder. In this first study to examine comorbid drinkers’ real-time assessment of alcohol versus non-alcohol drinking, drinking to intoxication in individuals with AUD was more closely associated with enhancement of positive affect than with relief of negative affect and observed in both those with and without concomitant depressive disorder. This real-world drinking study adds external validity to prior controlled laboratory studies suggesting the continuation of the reward-sensitive stage of drinking in the addiction process, even in persons with high negative affect by virtue of depressive disorder and suggests a co-existence, rather than progression from positive to negative reinforcement, in young adults most prone to negative affect-driven drinking by virtue of AUD and DEP comorbidity.