Ethanol consumption prior to cocaine self-administration reduced vulnerability to cocaine reinforcement in socially subordinate female and male monkeys

Abstract

Rationale:

Although ethanol consumption is ubiquitous among individuals who use cocaine, most preclinical work investigating factors that contribute to the development of problematic cocaine use do not incorporate ethanol into their experimental designs. Given that only a subset of individuals who try cocaine go on to develop a cocaine use disorder (CUD) research is needed to identify factors, such as ethanol consumption, that may influence vulnerability to cocaine reinforcement.

Objectives:

Thus, this study aimed to determine how a history of ethanol self-administration and exposure immediately prior to the first experience with cocaine self-administration influenced the potency of cocaine to function as a reinforcer in socially housed male and female cynomolgus monkeys.

Methods:

For these experiments, one group of monkeys (n=7) were trained to self-administer up to 1.5 g/kg of ethanol prior to cocaine self-administration while another group of monkeys (n=13) remained ethanol-naïve through the study. Acquisition of cocaine self-administration was studied in both groups of monkeys under a fixed-ratio schedule of reinforcement where ascending doses of cocaine were substituted for food pellets. Cocaine ED50 values were examined statistically.

Results:

The results showed that subordinate monkeys that self-administered ethanol prior to cocaine self-administration required higher cocaine doses to function as a reinforcer compared with subordinate monkeys not exposed to ethanol.

Conclusions:

One possible explanation for this finding is that ethanol, due to its acute anxiolytic effects, mitigated the effect of chronic stress in subordinate monkeys and thereby blunted the reinforcing effects of initial cocaine exposure. Future research is needed to examine whether reducing anxiety in chronically stressed individuals can modify the initiation and continued use of cocaine.

INTRODUCTION

Drug use is a major public health problem that can negatively influence health outcomes and social and economic welfare. In the U.S., cocaine use is increasing, which has corresponded with a 3-fold increase in cocaine-related overdose deaths between 2012 and 2018 [1, 2]. In addition, most individuals who use cocaine are polysubstance users which means they often use more than one drug together or within short succession. Although substances such as opioids, nicotine, and marijuana are often co-used with cocaine, the focus of these experiments will be cocaine and alcohol co-use [3–5]. This is because the co-use of cocaine and alcohol is ubiquitous in individuals who use cocaine and estimates suggest that up to 90% of individuals who use cocaine also consume alcohol [6, 7]. Even though co-use of alcohol with cocaine is the clinical reality, little research has examined how ethanol self-administration may influence cocaine reinforcement.

CUD is a multifactorial disorder and vulnerability to cocaine use likely results from complex interactions between environmental factors, the characteristics of the individual, and the pharmacology of the drug. As a result, only a subset of individuals who try cocaine will go on to develop a CUD [8]. Given that there are still no FDA-approved treatments for CUD, it is vital to conduct research to identify factors that may exacerbate or ameliorate vulnerability to cocaine use. One such factor may be alcohol consumption. In fact, one study in high schoolers found that 84% of cocaine users reported using alcohol before ever trying cocaine [9]. Also, another study in adolescents found that 70% of individuals reported cocaine use for the first time during alcohol intoxication [10]. Although epidemiological studies suggest that initial exposure to legal drugs such as alcohol is associated with an increased risk of using illicit drugs such as cocaine, these studies are often correlational and are unable to determine whether alcohol consumption is increasing vulnerability to cocaine reinforcement or if other factors are increasing vulnerability to using both legal and illegal drugs [11, 12]. While studies in laboratory animals are better able to determine whether ethanol exposure prior to cocaine self-administration increases vulnerability to cocaine reinforcement, results have been mixed.

In one study, male rats that self-administered higher amounts of ethanol across 36 sessions, self-administered more cocaine during an initial exposure [13]. In another study, male and female mice exposed to I.P. injections of ethanol across 2 weeks obtained a higher number of cocaine infusions [14]. However, both studies used only a single dose of cocaine making it impossible to know how increased cocaine infusions at that particular dose were related to vulnerability to cocaine reinforcement. In another study, male rats were exposed to intermittent ethanol self-administration across seven weeks and then acquisition of cocaine reinforcement was studied using an ascending dose procedure. Under these conditions, no relationship was found between alcohol pre-exposure and cocaine self-administration [15]. Additionally, one study in male rhesus monkeys found that after 9 months of self-administering up to 2.0 g/kg of ethanol there were no differences in cocaine acquisition when compared to control (i.e., alcohol-naïve) monkeys nor was the amount of ethanol consumed predictive of vulnerability to cocaine reinforcement [16].

It is important to note though that in all three rodent studies the animals were given at least two weeks between ethanol exposure and cocaine self-administration. Moreover, in the one study using nonhuman primates, access to ethanol and cocaine were separated by at least 3 hours to eliminate pharmacodynamic and pharmacokinetic interactions between ethanol and cocaine. However, studies suggest that these interactions may play a critical role in alcohol and cocaine co-use [4]. In fact, two common reasons given by individuals with CUD for their co-use of alcohol is to maintain a better high and to mitigate feelings of anxiety and depression caused by cocaine [4], [17]. Furthermore, one recent clinical report suggested that the most common pattern of cocaine and alcohol co-use involved simultaneous use or use of both drugs in close succession [18]. It is also important to note that most previous studies were conducted using male, individually or pair housed, laboratory animals [13, 15, 16]. Thus, the effect that sex or social housing conditions may play on the interaction between ethanol and cocaine is largely unknown.

Given that the acute interactions between ethanol and cocaine may play a role in increased vulnerability to cocaine use, this study had several aims. The first aim was to examine whether monkeys who had a history of orally self-administering ethanol (greater than 6 weeks) and had access to ethanol immediately prior to cocaine self-administration sessions acquired cocaine reinforcement at lower doses than monkeys with no history of ethanol self-administration, suggesting greater vulnerability to cocaine reinforcement. Moreover, we examined whether cumulative ethanol intakes, ethanol intake prior to cocaine self-administration, and the rate of ethanol consumption prior to cocaine self-administration influenced vulnerability to cocaine reinforcement. Additionally, variables related to the environment and characteristics of the organism may play a role in vulnerability to cocaine reinforcement. As a result, this study also aimed to examine whether social rank and sex differentially influenced vulnerability to cocaine reinforcement with and without ethanol self-administration.

Methods

Monkeys.

Twenty drug-naïve cynomolgus macaques served as subjects. Seven monkeys (4 dominants and 3 subordinates) were trained to self-administer ethanol while the remaining 13 (6 dominants and 7 subordinates) were ethanol-naïve throughout the study. All monkeys were housed in same-sex social groups of 4 and social rank was determined according to the outcomes of agonistic encounters as described previously [19–21]. These social ranks were well-established, and the monkeys were housed together for more than 12 months before cocaine acquisition occurred. They continued to be socially housed throughout the study. For this study, the number 1- and 2-ranked monkeys were considered dominant while the number 3- and 4-ranked monkeys were classified as subordinate. This was done to maximize statistical power. Not all monkeys in a pen were used in this study. The sex and social rank of the ethanol-naïve monkeys are shown in Table 1 and the sex and social rank of the ethanol self-administration monkeys can be found in Table 2. At the start of this study, all monkeys were naïve to drugs except for infrequent exposure to ketamine (I.M.), used for veterinary procedures and 1.5% isoflurane for PET studies in some of the monkeys. All monkeys were initially trained to respond on a concurrent fixed-ratio schedule of 1- vs. 3-food pellets prior to the start of the current study and those behavioral data from some of these monkeys have been previously published [22]. Moreover, because data related to the ethanol-naïve group of monkeys was historical, some of the data related to cocaine acquisition has been previously published [23]. The menstrual cycle in all females was monitored daily by vaginal swabs.

TABLE 1:

Subject number, sex, social rank, and ED50 values during cocaine acquisition in ethanol-naïve monkeys.

Monkey	Sex	Social Rank	Cocaine ED50 (mg/kg)
F-8554	Female	Dominant	0.065
F-8540	Female	Subordinate	0.006
F-8548	Female	Subordinate	0.007
F-8555	Female	Dominant	0.162
F-8537	Female	Dominant	0.005
F-8557	Female	Subordinate	0.005
F-8531	Female	Subordinate	0.005
F-8549	Female	Subordinate	0.002
M-8506	Male	Dominant	0.020
M-8558	Male	Subordinate	0.006
M-8503	Male	Dominant	0.006
M-8507	Male	Subordinate	0.019
M-8502	Male	Dominant	0.006

TABLE 2:

Subject number, sex, social rank, EtOH data and ED50 values during cocaine acquisition in ethanol self-administration monkeys.

Monkey	Sex	Social Rank	Cumulative EtOH intake (g/kg)	EtOH SA sessions	EtOH intake (g/kg)	EtOH rate (g/kg/minute)	Cocaine ED50 (mg/kg)
F-8581	Female	Dominant	113.17	90	1.42 ± 0.09	0.05 ± 0.02	0.017
F-8580	Female	Dominant	121.58	112	1.01 ± 0.40	0.04 ± 0.008	0.018
F-8609	Female	Subordinate	61.36	98	0.65 ± 0.19	0.02 ± 0.01	0.063
F-8579	Female	Subordinate	188.26	163	1.35 ± 0.44	0.03 ± 0.01	0.020
M-8676	Male	Dominant	78.55	64	1.14 ± 0.31	0.02 ± 0.01	0.015
M-8681	Male	Dominant	47.13	62	0.70 ± 0.20	0.02± 0.004	0.018
M-8682	Male	Subordinate	56.99	90	0.41 ± 0.15	0.007± 0.003	0.065

Ethanol-exposed and ethanol-naïve monkeys were group housed (n=4/cage) in stainless-steel cages, in a temperature- and humidity-controlled room maintained on a 14-hour light/10-hour dark cycle (lights on between 6:00 AM and 8:00 PM). Monkeys were fed standard laboratory chow (Purina LabDiet 0549, St Louis, MO) and maintained healthy body weights slightly below free-feeding weights. They were enriched daily with fresh fruits and/or vegetables and while in the homecage, water was available ad libitum. Environmental enrichment was provided as outlined in the Animal Care and Use Committee of Wake Forest University Non-Human Primate Environmental Enrichment Plan, including chew toys, mirrors, music, and foraging feeders. Animal housing, handling, and experimental protocols were performed per the 2011 National Research Council Guidelines for the Care and Use of Mammals in Neuroscience and Behavioral Research and were approved by the Animal Care and Use Committee of Wake Forest University.

Catheter implantation.

For the cocaine self-administration studies, each monkey was surgically implanted with a chronic indwelling intravenous catheter and subcutaneous vascular access port (VAP) under sterile conditions. Details of this surgery can be found in previously published work [24]. All monkeys were given at least one week to recover from the surgery before experimental sessions began.

Apparatus.

All monkeys, fitted with aluminum collars, were trained to sit in a primate restraint chair (Primate Products, Redwood City, CA). Cocaine self-administration experiments were conducted in ventilated, sound-attenuating primate chambers (1.5 × 0.76 × 0.76 m; Med Associates, St. Albans, VT). Ethanol self-administration experiments were conducted in the same primate chambers, but these chambers were equipped with a fluid distribution system designed to provided measured access to fluids. This system comprised of a fluid reservoir mounted to a strain-gage scale, delivery tube from the reservoir through the front of the chamber to a reinforced pipe and standard Lixit^®, and a control box which is attached to the scale. The control box contained a microcontroller Arduino Uno and a custom printed circuit board for receiving and sorting the input signal from the scale. During ethanol and cocaine self-administration sessions, white noise was played to mask any potentially obstructing sounds from outside the experimental room. A description of the operant chamber can be found in [24].

Ethanol self-administration procedure:

The procedure used to establish ethanol self-administration was similar to the procedure described by Katner et al. [25]. Briefly, each day prior to food-maintained responding under a concurrent food-food choice paradigm, the monkeys were given access to a sipper tube within the operant chamber and were able to drink a 6% Gatorade Zero Powder ^® solution mixed with water. During these sessions, no operant behavior was required and apart from the taste of ethanol, no discriminative stimuli were associated with fluid delivery. The initial solution provided corresponded to the volume of 1.5 g/kg (approximately 6 standard drinks) of a 4% ethanol solution and served as baseline sessions. Each day the monkey was given 1-hour to consume the solution and following the hour any remaining liquid was removed, and the monkey was able to respond on a concurrent schedule of food availability. Ethanol was introduced gradually across daily sessions during which the concentration (%) and/or amount (g/kg) of ethanol in the Gatorade ^® was increased. A detailed account of this procedure can be found in Table S1. This was done until the monkeys were consistently consuming a 4% ethanol solution with Gatorade^® and had access to 1.5 g/kg of this solution. The dose and concentration of ethanol was selected based on previous work that demonstrated that macaques readily consumed this combination and reached behaviorally relevant blood ethanol concentrations 90 minutes following the start of the session (>80 mg/dl) [26]. The monkeys were not water or food restricted during this study.

Ethanol self-administration sessions occurred 5 days a week and there were no somatic signs of withdrawal 24 or 48 hours after the last drinking session of the week (data not shown). This procedure was adapted from [27] and assessments occurred prior to the start of the study and following completion of the study. There was variability in the number of ethanol self-administration sessions prior to cocaine self-administration (Table 2) due to differences related to the length of time needed to train the monkey to drink the ethanol solution, differences in the number of sessions needed to stabilize food-maintained responding (mean reinforcement frequency ± 20% for three consecutive sessions) and differences in scheduling catheter implantation surgeries. As a result, although all monkeys had access to ethanol for at least 6 weeks prior to the start of this study, some monkeys self-administered ethanol for longer periods of time (Table 2). Food-maintained responding was studied immediately after oral ethanol self-administration to verify that when compared to the baseline sessions, ethanol did not disrupt operant responding. In all seven monkeys, ethanol did not disrupt food self-administration (Fig. S1). In all 7 monkeys, ethanol was available throughout the entire study.

Cocaine self-administration procedure:

After at least six weeks of ethanol self-administration followed by food-maintained responding, the conditions were changed from a concurrent schedule of 1- vs. 3- pellets [22] to a simple FR schedule of reinforcement following 1-hour access to ethanol self-administration. FR values ranged from 5–30 (Table S2). The historical data collected from ethanol-naïve monkeys also occurred after the monkeys responded under a concurrent schedule of 1- vs. 3- pellets for at least six weeks prior to cocaine self-administration. No ethanol was self-administered by these monkeys and some of the data collected during cocaine self-administration in ethanol-naïve monkeys have been previously published [23]. The methodology for establishing cocaine reinforcement has been previously described (see [22]). Briefly, a white light above a response switch signaled food or cocaine availability and completing the FR requirement resulted in the delivery of a 1.0-g-banana-flavored food pellet or a 10-second infusion of saline or cocaine. The delivery of either stimulus was following by a 10-s timeout (TO) and sessions ended after 30 reinforcers or 60 min, whichever occurred first.

After catheter implantation, food-maintained responding was reestablished and then saline was substituted for food pellets until responding was extinguished (mean reinforcement frequency decreased by >80% of food-reinforced responding for three consecutive sessions). After re-establishing food-maintained responding, different doses of cocaine were substituted for the food pellet reinforcer in ascending order (0.001–0.1 mg/kg/injection), which increased in half-log units. Each dose was available for at least three sessions and until responding was deemed stable (mean reinforcement frequency ± 20% for three consecutive sessions). The dose was not changed if there were any trends (increases or decreases in responding) in the data. Food-maintained responding was re-established for 2–5 sessions between cocaine doses. Sessions ended after 30 reinforcers or 60 mins, whichever occurred first. The same switch was used for food, saline, and cocaine self-administration.

Drugs:

(−)Cocaine HCl, supplied by the National Institute on Drug Abuse (Bethesda, MD), was dissolved in sterile 0.9% saline. Cocaine was delivered over 10-sec (~ 1 mL/10-sec); different doses were studied by changing the drug concentration. All drug doses are expressed as the salt form. 190 Proof ethyl alcohol was supplied by the Wake Forest University School of Medicine and was diluted into a 4% ethanol solution for consumption.

Data Analyses:

During each ethanol self-administration session, the total amount of ethanol drank (g/kg) was recorded. Repeated measures ANOVAs demonstrated that the amount of ethanol consumed, and the rate of consumption did not vary significantly between conditions where saline was available, food was available, or cocaine was available following ethanol self-administration sessions (p>0.05) (Table S3). Because of this, an average of the ethanol intakes during the entire cocaine acquisition procedure were taken to capture average ethanol intakes. Moreover, the average rate of consumption (g/kg drank per minute) was assessed during cocaine acquisition (see Table 2). Univariate ANOVAs were used to evaluate if there were any differences in average ethanol intakes and rate of consumption as a function of social rank or sex during acquisition in the ethanol self-administration monkeys (n=7). Differences in cumulative ethanol intakes and the number of ethanol self-administration sessions prior to the acquisition procedure as a function of rank and sex were also assessed with univariate ANOVAs. Cumulative ethanol intakes and the number of ethanol sessions were calculated using all days where ethanol was available for self-administration including sessions shown Table S1 where less than 1.5 g/kg of a 4% ethanol solution was available. Although this study was powered to evaluate sex or social rank differences, it was underpowered to evaluate interactions between sex and social rank. As a result, statistical tests were run separately for the two variables.

For the cocaine self-administration data, the rate of saline extinction was compared between ethanol-naïve and ethanol-experienced monkeys as a function of social rank or sex to examine whether differences in cocaine acquisition could be due to ethanol impairing task learning. Moreover, rates of food-maintained responding were compared. The primary dependent variable for cocaine acquisition was cocaine ED50 value (Tables 1 and 2). The cocaine ED50s were calculated from the ascending limb of the dose-response curve and were determined by calculating the mean number of injections received on the ascending limb of the dose-effect curve relative to the lowest dose tested and the dose at the peak of the dose-effect curve. This mean was then placed into a formula in Excel to determine at which dose this average number of injections would be earned for each monkey. Because the sample size for the ethanol-exposed and ethanol-naïve groups were different a Breusch-Pagan test for Heteroskedasticity was conducted. This was not significant (X² (1), N = 20) =2.24, p=0.135. Moreover, z-scores were calculated to identify any outliers in the dataset. No outliers were identified. Thus, to determine whether there was a difference in cocaine acquisition between ethanol-naïve monkeys and ethanol-exposed monkeys as a function of social rank or sex, two-way ANOVAs were run. As above, we were powered to evaluate sex or social rank but not sex and social rank across condition (ethanol group vs ethanol-naïve group) and thus, statistical tests were run separately for the two variables.

In addition, in the monkeys self-administering ethanol, follow-up analyses were conducted to examine whether average ethanol intakes and rates of ethanol consumption during cocaine acquisition sessions predicted cocaine ED50 values. These analyses were multiple, simultaneous, regressions where all predictors were entered into the regression equation at once. Cumulative ethanol intakes and the number of ethanol self-administration sessions conducted prior to the start of cocaine acquisition were also evaluated as predictors of ED50 values. Social rank and sex were included in these models and the reference groups were dominant monkeys and male monkeys. For all statistical tests, significance was set at an alpha of 0.05 and all tests were analyzed with SPSS.

Results

Analyses conducted in the monkeys that self-administered ethanol found no significant effect (p>0.05) of social rank or sex on the average amount of ethanol consumed or the rate of ethanol consumption during cocaine acquisition (Table 2). There were no significant (p>0.05) sex or social rank differences in cumulative ethanol intakes or the number of ethanol self-administration sessions prior to cocaine acquisition (Table 2). Prior to cocaine acquisition, there were no differences (p>0.05) in baseline number of food presentations between ethanol-naïve (M=27.69, SEM=1.39) and ethanol self-administration monkeys (M=27.27, SEM=1.91) when sex or social rank was included in the model (Table S2). When saline was substituted for food, there were no group differences (p>0.05) in the number of sessions needed to extinguish responding for the ethanol-naïve monkeys (M=6.65, SEM=0.90) and for the ethanol self-administration monkeys (M=9.13, SEM=1.21). No effect of sex or social rank was noted (p>0.05) (Table S2).

To examine acquisition of cocaine reinforcement, ascending doses of cocaine were substituted for food. Individual dose-effect curves are shown in Figures 1 and 2. When social rank was incorporated into the analysis, there was a significant interaction between rank and condition (F(1,19)=7.57, p=0.014, ηp²=0.321), such that subordinate monkeys drinking ethanol prior to access to cocaine had significantly higher ED50 values (0.051±0.013 mg/kg) when compared to subordinate ethanol-naïve monkeys (0.006±0.009 mg/kg) (p=0.013, ηp²=0.327). (Figure 3). There was also a trending effect such that within the ethanol-naïve group, dominant monkeys (0.033±0.010 mg/kg) had marginally higher ED50 values than subordinate monkeys (p=0.053, ηp²=0.214). Furthermore, within the ethanol group, subordinate monkeys had marginally higher ED50 values than dominant monkeys (0.017±0.012 mg/kg), (p=0.078, ηp²=0.182) (Figure 3). When sex was included as a variable of interest, there was no significant effect of condition (ethanol-naïve group vs ethanol-exposed group) in terms of ED50 values during the cocaine dose-response curve determination (p>0.05).

FIGURE 1:

Individual cocaine acquisition dose-response curves under a fixed-ratio schedule of reinforcement in ethanol-naïve dominant (A) and subordinate (B) monkeys. Female monkeys have numbers beginning with F while male monkeys have numbers beginning with M. Each data point represents the mean ± the standard deviation from the last three sessions.

FIGURE 2:

Individual cocaine acquisition dose-response curves under a fixed-ratio schedule of reinforcement in ethanol-exposed dominant (A) and subordinate (B) monkeys. Female monkeys have numbers beginning with F while male monkeys have numbers beginning with M. Each data point represents the mean ± the standard deviation from the last three sessions.

FIGURE 3:

Average cocaine ED50 values (mg/kg) during the acquisition procedure in dominant (open bars) and subordinate (hatched bars) monkeys as a function of condition (ethanol-naïve or ethanol-exposed groups). Error bars represent the mean ± the standard error of the mean. * Asterisks depict significant p values.

Further analyses investigating the ethanol group found that when sex and social rank were included in the model, cumulative ethanol intakes were significantly negatively related to ED50 values (t(6)=−5.03, p=0.015), such that higher cumulative ethanol intakes were related to lower cocaine ED50 values (i.e., greater potency to function as a reinforcer). Social rank was a significant positive predictor in this model t(6)=8.34, p=0.004), while sex was not a significant predictor (p>0.05) (Table 3). Although trending (t(6)=−2.42, p=0.094), the average ethanol intake during cocaine acquisition was not a significant predictor of cocaine ED50 values (Table S4). The rate of ethanol self-administration during cocaine acquisition sessions and the number of ethanol self-administration sessions prior to cocaine acquisition were also not significant predictors of cocaine ED50 values (p>0.05) (Table S5 and Table S6).

Table 3:

Linear regressions and significance levels between cumulative ethanol intakes and cocaine ED50 values in the monkeys self-administering ethanol prior to the self-administration sessions. Sex and social rank were also included as predictors.

Predictor	Standardized B	P
Cumulative ethanol intake	−0.676	0.015 *
Social rank	0.912	0.004 *
Sex	0.202	0.226

^*Asterisks depict significant p values.

Discussion

Previous preclinical studies investigating the relationship between vulnerability to cocaine reinforcement and ethanol consumption have had mixed results with some studies showing no relationship between a history of ethanol self-administration and later cocaine self-administration and some studies showing an increase in cocaine self-administration following a history of ethanol exposure [16–19]. To our knowledge, no previous studies have investigated how oral ethanol self-administration prior to and during cocaine acquisition modulates the reinforcing effects of cocaine. However, clinical studies suggest that most individuals who try cocaine have a history of ethanol consumption and were actively intoxicated while first using cocaine [9, 10]. As a result, this study aimed to more closely model the human condition where the monkeys not only had a history of oral ethanol self-administration but were also able to self-administer ethanol for one hour prior to cocaine self-administration. Furthermore, this study incorporated sex and social rank as variables of interest which have previously not been assessed in relation to ethanol and cocaine co-use [13–16].

One main finding was that ethanol self-administration decreased vulnerability to cocaine reinforcement specifically in subordinate monkeys. In these social groups, subordinate monkeys lose all fights, get groomed the least, and have the least access to food/treats [28, 29]. Substantial research has supported the premise that social subordination is a model of chronic social stress [30]. In fact, subordinate monkeys are more susceptible to numerous diseases such as respiratory infection and atherosclerosis which is consistent with findings in humans that suggest a relationship between social condition and vulnerability to disease [31–34]. Studies have shown that subordinate monkeys have heavier adrenal glands and greater adrenal responsiveness to an adrenocorticotrophic hormone challenge when compared to their dominant counterparts [28, 35, 36]. Furthermore, one meta-analysis examining social rank-related differences in cortisol levels among primates concluded that subordinates generally exhibited higher cortisol levels which are indicative of higher stress [37].

In this study, one supported interpretation of the differences between the dominant and subordinate ethanol-naïve monkeys is that chronic social stress in the subordinate monkeys increased vulnerability [28, 29]. However, it should be noted that differences in dominant and subordinate monkeys were only marginal due to a small sample size (p=0.051). Despite this, evidence suggests that chronic environmental stressors can enhance the reinforcing strength of cocaine by increasing the activity of the brain’s dopaminergic systems [38, 39]. However, ethanol self-administration may reverse this phenotype in subordinate animals due to its acute anxiolytic effects and studies in humans and laboratory animals have shown that acute exposure to low-to-moderate doses of ethanol reduced anxiety and attenuated responses to stressors [40–42]. Thus, it is possible that by acutely decreasing stress prior to the cocaine self-administration sessions, ethanol mitigated the effects that chronic stress has on sensitivity to the reinforcing effects of low doses of cocaine.

This premise has been supported by both neurochemical and behavioral studies. For example, one study examining the effects of diazepam on stress-induced dopamine release in rats showed that diazepam attenuated stress-related increases in extracellular dopamine [43]. This, in turn, could dampen the effect that stress has on the reinforcing effects of cocaine. Moreover, numerous studies in rodents and monkeys have shown that both benzodiazepine [44–47] and non-benzodiazepine anxiolytics can reduce cocaine self-administration [48, 49]. Another important note is that ethanol had no effect on cocaine self-administration in dominant monkeys, suggesting that the social environment may play an important role in how anxiolytic compounds, such as ethanol, impact vulnerability to cocaine reinforcement. This topic should be further investigated.

These findings, however, should be interpreted with the caveat that no direct physiological or behavioral measures were taken in this study to verify that the subordinate monkeys were chronically stressed or that ethanol was acutely anxiolytic. As a result, future studies should be conducted to examine whether various measures of chronic stress are acutely reduced by ethanol self-administration specifically in subordinate monkeys. Moreover, studies should be conducted to determine whether other anxiolytic compounds with shared neuropharmacological effects with ethanol can produce similar outcomes to those reported in this study. Overall, these results would further elucidate whether reducing anxiety in chronically stressed individuals can modify the initiation and continued use of cocaine [50].

An additional finding from the ethanol self-administration group was that higher cumulative ethanol intakes were predictive of lower cocaine ED50 values during acquisition. The number of ethanol self-administration sessions, however, was not related to acquisition cocaine ED50 values suggesting that the amount of ethanol consumed was more important than the length of time the laboratory animal self-administered ethanol prior to cocaine acquisition. Moreover, although the average ethanol intake during acquisition immediately prior to cocaine self-administration did not reach significance, there was a trending relationship such that higher average ethanol intakes during acquisition predicted lower cocaine ED50 values.

Together, these results demonstrated that higher historical ethanol consumption increased sensitivity to the reinforcing effects of cocaine. This finding is consistent with some previous work in rodents demonstrating that ethanol exposure enhanced cocaine self-administration [13, 14]. However, these findings are not consistent with previous work in monkeys which found no relationship between the reinforcing potency of cocaine and cumulative ethanol intakes despite average cumulative ethanol intakes being higher than the ones in this study [16]. One possible reason for lack of concordance with the earlier nonhuman primate study was methodological. In this study, ethanol was self-administered for more than 6 weeks prior the cocaine availability and was also available immediately prior to cocaine self-administration, allowing for pharmacokinetic and pharmacodynamic interactions to occur. It is possible that ethanol intakes only predict cocaine acquisition in the presence of these interactions. However, because the ethanol monkeys had access to ethanol both prior to and during this study, it is not possible to distinguish whether the effect of ethanol self-administration on vulnerability to cocaine reinforcement occurred because of a history of ethanol drinking, acute ethanol drinking, or a combination of the two.

Some additional limitations should be noted. Because the data from the ethanol-naïve monkeys were historical data, these monkeys did not have access to a vehicle solution prior to and during cocaine acquisition. This could be confounding since the ethanol-naïve monkeys did not self-administer a calorific fluid prior to cocaine self-administration. The ethanol-naïve monkeys were also run at a different time temporally than the ethanol-exposed monkeys which could have contributed to some group differences. Although these differences in experimental history between the ethanol-naïve and ethanol-exposed monkeys may have influenced the findings of this study, there were no noted differences in food-maintained responding or the rate of saline extinction between the two groups of monkeys prior to cocaine acquisition and no differences in cocaine acquisition between dominant ethanol-naïve and ethanol-exposed monkeys. However, it is impossible to know whether drinking a caloric solution alone would have influences cocaine self-administration in the ethanol-naïve group. Thus, the findings of this study should be interpreted in the context of this caveat.

Moreover, for this study, ethanol self-administration occurred prior to cocaine self-administration while a recent clinical report suggested that patterns of cocaine and alcohol co-use usually involve simultaneous use [18]. As a result, future studies should further elucidate how the pattern of ethanol and cocaine co-use influence behavioral outcomes. In addition, when cocaine and ethanol are used simultaneous or in close succession the metabolite cocaethylene is formed [24, 51]. Since cocaethylene formation was not measured in this study, it remains an empirical question whether differences in sensitivity to cocaine reinforcement are related to this metabolite.

Additionally, previous studies using cynomolgus monkeys have demonstrated that in male monkeys, subordinate monkeys have higher daily ethanol intakes when compared to dominant monkeys [52]. Furthermore, one study in cynomolgus monkeys found that males drank approximately 1.5-fold more than females [53]. While there were no significant sex or social rank differences in ethanol intakes and rates in this study, this is likely due to methodological differences where in the previous studies the monkeys had access to ethanol in the homecage for a significantly longer portion of the day (16–22 hours per day) [52, 53]. One important point is that because ethanol was mixed into a sweetened solution in this study, no conclusions can be made about the reinforcing effects of ethanol. However, future studies should be conducted to determine whether there are differences in the reinforcing strength of ethanol as a function of sex or social rank under conditions where the monkeys self-administer ethanol outside of the homecage and under more limited conditions (1 hour per day).

While epidemiological studies suggest that initial exposure to legal substances such as alcohol are associated with an increased risk of using illicit drugs such as cocaine, the present study showed that in chronically stressed monkeys, ethanol self-administration may be protective against cocaine reinforcement when compared to non-drinking subordinate monkeys [11, 12]. However, within the monkeys who drank ethanol, greater cumulative ethanol intakes were related to greater sensitivity to cocaine reinforcement. These findings demonstrate that the effect of ethanol self-administration on cocaine reinforcement varies based on social variables, especially exposure to chronic social stress. Future research is needed to examine how ethanol self-administration may modify other components of the cocaine use cycle, including the maintenance of cocaine self-administration and efficacy of various behavioral and pharmacological treatments as a function of social rank.

Data Availability Statement

Data will be available upon request.