Alcohol and Oxytocin: scrutinizing the relationship

Abstract

The initial enthusiasm towards oxytocin (OXT) as a potential treatment for alcohol use disorder has been recently tempered by recognizing existing gaps in literature and the recent appearance of a relatively small number of clinical studies with negative outcomes. On the other hand, several new studies continue to support the OXT system’s potential for such treatment. In this review, we thoroughly analyze existing literature assessing both alcohol’s effects on the OXT system and OXT’s effects on alcohol-related behaviors. Both rodent and clinical research is discussed. We identify areas that have been studied extensively and those that have been undeservingly understudied. OXT’s potential effects on tolerance, withdrawal, craving, anxiety and social behaviors, and how these processes ultimately affect alcohol consumption, are critically explored. We conclude that while OXT can affect alcohol consumption in males and females, more comprehensive studies on OXT’s effects on alcohol-related tolerance, withdrawal, craving, anxiety and social affiliations in subjects of both sexes and across several levels of analyses are needed.

1. Introduction

Alcohol use disorder (AUD) remains a recurrent, devastating health condition with limited treatment options. There is rapidly rising interest in the oxytocin (OXT) system as a potential target for development of treatments for AUD (Lee et al., 2013; Lee and Weerts, 2016; Leong et al., 2018; McGregor and Bowen, 2012; Pedersen et al., 2013). Our analysis of 1650 PubMed publications featuring keywords “oxytocin” and “alcohol” identified 131 publications mentioning interactions between them. There is a particularly sharp increase in the number of these publications with human subjects in recent years, reflecting the increased interest in oxytocin (OXT) as a treatment of AUD-related conditions (Figure 1). Initially, several highly impactful publications demonstrated the ability of intranasally (IN) administered OXT to decrease symptoms of alcohol withdrawal and craving in humans (Pedersen, 2017; Pedersen et al., 2013). However, these studies were followed by an accumulation of negative findings, in which OXT was found to be an ineffective AUD treatment (Melby et al., 2019; Melby et al., 2020b; Vena et al., 2018). Some researchers also referred to the sparsity of studies in female subjects, but argue that because several of said studies did not observe an effect of alcohol dependence on the OXT in females, clinical trials with OXT and AUD should only be performed in male subjects (Hansson and Spanagel, 2021). While the analysis of literature confirms the predominance of studies using male subjects (Figure 2), studies have observed effects of alcohol on the OXT system and effects of OXT on alcohol intke in both male and female subjects (Tables 1 and 2). Nevertheless, the accumulation of negative data indeed suggests that OXT treatment is not uniformly effective. Therefore, perhaps, more complex treatment strategies with OXT will need to be developed, suggesting that preclinical and clinical studies should continue to include both male and female subjects. To aid such development, we critically review existing literature on both the effects of alcohol on the OXT system, and the contribution of the OXT system to alcohol-related behaviors, including alcohol consumption, paying close attention to whether the studies were performed in male and/or female subjects and whether sex differences were observed.

Figure 1.

Changes in the number of publications mentioning interactions of oxytocin and alcohol per year over time by type of article. Note uneven time scale to illustrate rapid increase in recent publications. Type of article: Reviews, Original research on human subjects, Original research on non-human subjects.

Figure 2.

Changes in the number of publications mentioning interactions of oxytocin and alcohol per year over time by sex of subjects. Note uneven time scale to illustrate rapid increase in recent publications.

Table 1.

Preclinical studies on effects of OXT on alcohol intake and reward-related measures.

Phenotype	Effect	Subjects	Route/Method	Sex	Reference
Alcohol 2-bottle choice	Decrease	P rats	0.3 mg/kg repeated, IP, pre-exposure during adolescence	unknown	Knapp et al., 2010
Alcohol 2-bottle choice	Decrease	Wistar rats	1 mg/kg repeated, IP, during adolescence	Male	Bowen et al., 2011
Alcohol-spiked sweetener preference	Decrease	P rats	1 mg/kg, IP	Male & Female	McGregor and Bowen, 2012
Alcohol 2-bottle choice	Decrease	C57BL/6N mice	10 mg/kg, IP or 0.5 μg, ICV	Male	Peters et al., 2013
Conditioned Place Preference	Decrease	C57BL/6 mice	Carbetocin 6.4 mg/kg repeated, IP	Male	Bahi, 2015
Alcohol-spiked saccharine and ethanol-containing gelatin intake	Decrease	Sprague-Dawley rats	0.1–0.5 mg/kg, IP	Male	MacFadyen et al., 2016
Alcohol 2-bottle choice	Decrease	C57BL/6 mice	Viral overexpression of OXTR	Male	Bahi et al., 2016
Alcohol 2-bottle choice	Decrease	Wistar rats	1 μg, ICV	Male	Peters et al., 2017
Short access alcohol drinking & progressive ratio operant self-administration	Decrease	C57BL/6J mice	0.1–10 mg/kg, IP (specific effects at 0.1–0.3 mg/kg)	Male	King et al., 2017
Alcohol 2-bottle choice	Decrease	Prairie voles	1–10 mg/kg, IP	Male & Female	Stevenson et al., 2017a
Cue-induced reinstatement	Decrease only in dependent	Wistar rats	1 μg, ICV	Male	Hansson et al., 2018
Conditioned place peference	Increase	Swiss mice	Carbetocin, 6.4 mg/kg repeated, IP	Male	Rae et al., 2018
Operant self-administration	IN-induced decrease in dependent, IP-induced decrease in all	Wistar rats	0.125, 0.25 mg/kg, IP 1 mg/kg, IN	Male	Tunstall et al., 2019
Operant reinstatement	Decrease, higher sensitivity in females	C57BL/6J mice	0.5–1 mg/kg, IP	Male & Female	King and Becker, 2019
Alcohol 2-bottle choice	Increase in females	C57BL/6J mice	OXTR deletion	Male & Female	Rodriguez et al., 2020
Alcohol 2-bottle choice in social housing	Decrease	Prairie voles	3 mg/kg, IP, repeated	Male & Female	Walcott and Ryabinin, 2021
Alcohol 2-bottle choice in social housing	Decrease, varied across days	C57BL/6J mice	3 mg/kg, IP, repeated	Male & Female	Caruso et al., 2021

Table 2.

Human studies on effects of OXT on alcohol intake and reward-related measures

Phenotype	Effect	Subjects	OXT Route/Method	Sex	Reference
Craving	Decrease	Adults	0.5 μg/kg, IN, repeated	Males & Females	Pedersen et al., 2013
Alcohol consumption and AUD	Increase, partially sex-dependent	Adolescents	OXTR polymorphism rs53576	Males & Females	Vaht et al., 2016
Craving	Mixed effects	Adults	IN, 0.8 μg/kg	Males & Females	Mitchell et al., 2016
Alcohol dependence	Increase	Adults	OXT polymorphism rs6133010	Males	Yang et al., 2017
Number of heavy drinking days Number drinks per drinking day Craving	Decrease Decrease No effect	Adults	0.9 μg, IN, repeated	Males & Females	Pedersen, 2017
Cue reactivity	Decrease	Adults	0.5 μg/kg, IN	Males	Hansson et al., 2018
Regional connectivity, correlated with craving	Decrease	Adults	0.5 μg/hg, IN	Males	Bach et al., 2019
Stress-induced craving	No effect	Adults	IN, 0.8 μg/kg	Males	Flanagan et al., 2019
Cue-induced craving	No effect	Adults	IN, 0.4 or 0.8 μg/kg	Males	Stauffer et al., 2019
Number of days until relapse, amount of alcohol consumed per day, craving	No effect	Adults	maximum of 0.5 μg per day, IN repeated	Males & Females	Melby et al., 2020b

2. What this review does not cover

The OXT literature is vast: PubMed identifies 28,339 articles using this keyword. This review focuses on OXT, the OXT receptor (OXTR) and their interactions with alcohol. Although OXT has an affinity to the arginine vasopressin (AVP) receptor, discussing effects of AVP receptor activation is beyond the scope of this review. To keep this article focused, we will also omit a review of the organization of the OXT system. Likewise, we do not discuss the endogenous functions of OXT. We mention only briefly that besides the classic view of OXT as a hormone regulating parturition and lactation, and the more recent understanding of its critical involvement in regulation of social behaviors, it also contributes to regulation of behavioral and physiological responses to stressors, learning, thermoregulation, nociception, and food and fluid consumption. For these topics, the reader is referred to these outstanding and comprehensive review articles (Carter et al., 2020; Gimpl and Fahrenholz, 2001; Grinevich and Neumann, 2021; Jurek and Neumann, 2018; Lee et al., 2009; Leong et al., 2018; Peris et al., 2020). At this point, it is not entirely clear whether any of these functions are independent or interdependent.

We also do not thoroughly discuss brain penetration of OXT. In brief, the “fragile” consensus in the field is that although the half-life of OXT is only on the order of tens of minutes and its blood-brain-barrier penetrance low, increases in brain OXT are observed following its peripheral administration (Freeman et al., 2016; Lee et al., 2018; Neumann et al., 2013). This administration includes intranasal (IN) and intravenous (IV) routes, which are the preferred routes of administration (ROAs) in humans, but also include intra-peritoneal (IP) and subcutaneous (SC) ROAs, which are most commonly employed in rodents. The effects of peripheral administration may be aided by feed-forward effects, where peripheral administration of OXT, promotes endogenous OXT synthesis and release from the brain’s OXT centers (Neumann et al., 2013). The effects of IN and IP administration are also most likely aided by actions of active transport via the molecule RAGE (receptor for advanced glycation end products) (Yamamoto and Higashida, 2020; Yamamoto et al., 2019). While understanding these mechanisms is important when assessing behavioral functions of OXT, this discussion would be beyond the scope of a review focused on whether OXT can modulate AUD. Though recognizing the importance of ROA, we do mention the routes of OXT administration used when describing specific studies.

Although highly relevant, we also don’t discuss OXT’s interactions with the effects of other addictive drugs. The literature on OXT and alcohol is complex, and the reader is referred to published reviews on interactions of the OXT system with other addictive substances (Baracz and Cornish, 2016; Bowen and Neumann, 2017; King et al., 2020; Kovacs et al., 1998; Lee et al., 2016; Lee and Weerts, 2016; Leong et al., 2018; McGregor and Bowen, 2012; Pedersen, 2017).

Finally, we do not discuss review articles. There is a clear increase in review articles on the OXT system and alcohol (Figure 1). However, whenever a review article mentioned data that have not been published separately, we assessed it as an original investigation. Typically, such works contain less information on methods or scope of the study, but we treated them as another primary source of information nonetheless.

Here, we will first discuss studies elucidating the effects of alcohol on the OXT system, followed by those examining the effects of OXT on alcohol-related behaviors. Whenever the direction of association between OXT and alcohol consumption is not clear (for example, postmortem human studies), we attempt to clarify these effects based on other existing literature.

As a technical note: researchers from different laboratories describe administered doses in different units: in the International System of Units (SI), in moles, International Units (IU) designated by International Pharmacopeia, and on a per kilogram or per animal basis. To improve comparison between studies we have converted most doses into mg/kg (or μg/kg), approximating subject’s body weight (with the exception of intracranial injections and ex vivo studies).

3. Alcohol’s effects on the OXT system.

3.1. Alcohol and plasma OXT.

There is a substantial amount of evidence that a single alcohol exposure suppresses exogenously and endogenously-evoked increases in OXT release into the blood stream. While the earliest study on this subject did not detect significant effects of severe alcohol intoxication on nicotine-altered plasma OXT levels in male rats (Bisset and Walker, 1957), this study was followed by a relatively large series of findings showing that alcohol administration decreases plasma OXT. The demonstrations of these effects in rabbits and rats via an IV ROA (Fuchs, 1966, 1969; Fuchs and Wagner, 1963a, b) were quickly followed by similar clinical observations using IV or oral routes (Cobo, 1973; Coiro et al., 1992; Fuchs et al., 1982; Fuchs et al., 1968; Mennella and Pepino, 2006; Mennella et al., 2005; Wagner and Fuchs, 1968). These effects were functional, as the typical measured readouts of OXT’s effects in earlier studies were uterine activity and milk ejection. As such, there were originally more frequent studies in female than in male subjects.

These net effects of acute alcohol on circulating OXT levels are likely due to an alcohol-induced decrease in frequency of OXT spurts (Gibbens and Chard, 1976). Alcohol consumption also attenuated glucose test-induced increases in OXT in human males (Coiro et al., 1991). Similarly, 1–4 g/kg ethanol (IP) inhibited plasma OXT levels induced by administration of cholecystokinin or hypertonic saline in male rats (Blackburn et al., 1994) without affecting basal OXT levels in these subjects. Indeed, studies assessing effects of alcohol on basal (as opposed to evoked by other factors) plasma OXT levels, including recent investigations (Bershad et al., 2015; Dolder et al., 2017), often did not detect significant effects. On the other hand, injection of lower ethanol doses (0.05–1 g/kg, IP) increased these basal levels compared to saline controls in male Sprague-Dawley rats (Uvnas-Moberg et al., 1993). Taken together, the majority of studies indicate that acute alcohol exposure inhibits release of OXT induced by various factors in male and female human and non-human subjects. These studies also demonstrate that OXT levels typically normalized back to baseline levels following a decrease in blood alcohol levels.

The consistent inhibitory effects of alcohol administration and alcohol consumption on stimulus-evoked OXT release were followed by similar observations in ex-vivo hypothalamo-hypophyseal preparations from male rats. An early study showed that incubation of such preparations with alcohol resulted in decreased OXT release at lower in vitro doses (35–50 mM) and increased OXT release at higher doses (75–100 mM), an effect specific to alcohol, in that no effects of acetaldehyde incubation were observed (Hashimoto et al., 1985). Although OXT was not directly examined in isolated hypophyseal terminals following alcohol incubation, the release of AVP in these preparations was attributed to alcohol-induced decreases in calcium current and increases in BK channel activity (Dopico et al., 1996; Wang et al., 1991). Since the same terminals also contain OXT, it is highly likely that these direct mechanisms of alcohol’s action are contributing to decreased OXT release as well. In fact, in a later study, potassium-stimulated OXT release was lower in hypophyseal preparations from male rats in the presence of 10 mM or 75mM alcohol (Knott et al., 2000). Similarly to the in vivo studies discussed above, basal OXT levels in these preparations were not affected by the presence of alcohol (Knott et al., 2000). Additionally, enzymatic activity of aminopeptidases involved in hydrolysis of OXT from cortical synaptosomes of male BALB/c mice were also found to be regulated by ex-vivo alcohol treatment (Mayas et al., 2004). Theoretically, such effects could contribute to alcohol’s modulation of OXT levels, but these mechanisms have not been further explored.

Notably, the ex-vivo studies discussed above were performed only in male animals. Despite the lack of such studies in preparations from female animals, the similarities between these studies and those conducted in vivo strongly suggest that alcohol’s inhibition of OXT is a conserved effect occurring in both males and females.

In contrast to acute administration, there are much fewer published studies on the effects of prolonged or chronic alcohol exposure on plasma OXT levels in vivo. Lower OXT levels were found in men without AUD that were not restricted in alcohol consumption versus abstinent alcohol-dependent male patients (Marchesi et al., 1997). However, it is unclear whether these levels were lower in patients without AUD due to a lack of dependence or to the consumption of alcohol. Preclinical studies have shown that chronic exposure (3 weeks) to alcohol via a liquid diet attenuated alcohol-induced inhibition of OXT release from ex-vivo hypophyseal terminals of male Sprague Dawley rats (Knott et al., 2000), a finding indicating that tolerance to the endocrine effects of alcohol occurs, at least in part, in the hypophyseal terminals. Further studies found no difference between plasma OXT levels in male Wistar rats exposed to alcohol for 4 weeks via liquid diet versus controls (Da Silva et al., 2013) and no significant correlations between self-reported alcohol consumption in young male volunteers and plasma OXT levels (Betka et al., 2018). In a recent study, there was also no difference in OXT levels following testing for attachment responses in men with polysubstance use disorder (including alcohol) versus controls (Fuchshuber et al., 2020). These findings suggest that although acute alcohol tends to inhibit stimulus-evoked OXT release, tolerance in the peripheral components of the OXT system can prevent such an effect following repeated alcohol exposures.

3.2. Alcohol’s effects on central OXT and OXTR.

An early study performed in male Sprague-Dawley rats observed changes in morphology of neurons in the paraventricular nucleus of hypothalamus (PVN) and supraoptic nucleus (SON), the main sites of OXT production, following intragastric feeding of 2.4 g/kg of alcohol twice per day for 2 weeks, though this study did not specify neuronal type (i.e., OXT-expressing) (Hirvonen et al., 1966). Two later studies reported loss of OXT neurons specifically in SON of both male and female Wistar rats following an approximately year-long exposure to alcohol via a liquid diet (Madeira et al., 1993; Sousa et al., 1995). While prolonged alcohol-containing liquid diet can result in nutritional and hydration imbalance, these potential effects were taken into account by the inclusion of appropriate controls. There were also observations of compensatory increases in the size of remaining OXT neurons. Prolonged (6 months) withdrawal from this exposure resulted in reversal of the loss of these neurons and their enlargement (Sousa et al., 1995). A loss of PVN OXT neurons was also observed after 6 and 10 months of alcohol exposure via liquid diet in male Wistar rats, and there was a compensatory increase in OXT mRNA expression in remaining OXT neurons (Silva et al., 2002).

These findings correlated well with those from a study that examined OXT-immunoreactivity (OXT-ir) in PVN and SON of postmortem male brains, reporting decreased OXT-ir in SON of male chronic alcoholic patients compared to individuals without alcohol-related pathology (Sivukhina et al., 2006). On the other hand, there were no significant effects on PVN or SON OXT mRNA expression in male Wistar rats when the history of ethanol exposure was much shorter (4-week liquid diet ethanol exposure), suggesting that longer periods of ethanol access are needed to observe changes in hypothalamic OXT mRNA expression (Da Silva et al., 2013). In a human postmortem study, higher levels of OXT mRNA expression were observed in the prefrontal cortex of male subjects with AUD compared to non-dependent male controls (Lee et al., 2017).

Such ethanol-induced alterations in OXT production could, in turn, result in compensatory changes in OXTR expression within target sites. Adolescent intermittent 11-day exposure to high doses (4 g/kg) of intragastric alcohol resulted in OXTR-dependent decreases in social investigation of familiar rats and decreases in cell surface OXTR in hypothalamic samples in male, but not female, littermate Sprague-Dawley rats (Dannenhoffer et al., 2018). In contrast, when a similar study was performed in rats unfamiliar to each other, there were no effects on social interaction and no significant differences in OXTR expression in the central nucleus of amygdala (CeA) or lateral septum between alcohol and water-intubated rats of both sexes (Kim et al., 2019). While the forced nature of such alcohol exposure needs to be taken into cautious consideration (the intubation procedure itself caused significant changes in OXTR levels and novelty seeking in females, as well as changes in OXTR mRNA levels in the lateral septum of both sexes), these studies suggest that housing or social factors could modify effects of alcohol on the OXT system.

The use of other forced ethanol exposure models has revealed further region-specific effects on OXT and OXTR levels. An extensive investigation using an intense protocol of alcohol dependence-induction (seven weeks of intermittent alcohol vapor inhalation) reported a significant decrease of OXT mRNA expression and protein levels in PVN and SON of adult male Wistar rats (Hansson et al., 2018). While this decrease fluctuated in magnitude at subsequent time points of alcohol withdrawal, it remained significant out to 21 days. This decrease in OXT levels in PVN and SON was also accompanied by increased OXTR mRNA levels in several cortical areas, nucleus accumbens (NAC), caudate putamen (CPu), amygdala, and ventral hippocampus of rats in alcohol withdrawal. It also resulted in an increase in OXTR binding in cortical areas and CPu. The directionality of alcohol-induced changes in NAC OXTR mRNA levels varied depending on timepoint of withdrawal. Specifically, OXTR mRNA levels were lower than in controls immediately after and on the first day following the last session of intermittent alcohol exposure, with no difference on subsequent days. These levels were then increased in comparison to controls at 3 weeks after the last alcohol exposure. The same study reported increased OXTR and OXT binding in anterior cingulate, Brodman area 9, nucleus caudatus and ventral striatum in postmortem brains of male AUD subjects versus controls (Hansson et al., 2018). Similarly to the human literature, transcriptomic analysis of male mice on a C57BL/6J genetic background mice exposed to a 4-week intermittent alcohol 2-bottle choice procedure had decreased hypothalamic OXT mRNA levels (Zhou et al., 2019). This effect, however, depended on genotype. Specifically, in male mice that were lacking hypothalamic beta-endorphin due to a genome-wide deletion of neuronal proopiomelanocortin enhancer (consequently having lower basal OXT expression), alcohol consumption resulted in an increase in OXT mRNA levels, restoring these levels back to those of wildtype C57BL/6J mice (Zhou et al., 2019).

Remarkably, the decrease in OXT and OXTR levels in male rats and post-mortem alcohol-dependent men described in the previous paragraph were later not replicated in female rats and women (Hansson and Spanagel, 2021). This contradiction casted serious doubt on whether the effects of chronic alcohol on central OXT sites observed in male subjects occur in females.

However, an earlier study using female Sprague-Dawley rats showed that an alcohol liquid diet in combination with chronic nicotine administration (via minipump) during pregnancy resulted in decreased OXT levels in medial preoptic area and VTA, sites innervated by OXT centers, during parturition (McMurray et al., 2008). This effect was also accompanied by decreased maternal behaviors. While the effects of alcohol versus nicotine in this study are impossible to disentangle in this study, the sparsity of previously reported effects of nicotine on maternal behaviors suggests that these findings are driven by alcohol.

Additional studies have observed similar alcohol-induced alterations in central OXT levels in male and female prairie voles, a species known to display social (but not necessarily reproductive) monogamy and flexible strategies for mating behaviors (Carter and Getz, 1993; DeVries et al., 1997; Ophir et al., 2008). Furthermore, mechanisms identified as regulating social attachment in this species have been subsequently found to play a role in social attachments in humans, indicating that mechanisms regulating social behaviors in prairie voles have not only face, but also predictive validity for human social behaviors (Ebstein et al., 2009; Heinrichs et al., 2009; Insel et al., 1995; Meyer-Lindenberg et al., 2011). Moreover, prairie voles prefer alcohol solutions over water, when given access to them (Anacker et al., 2011; Ryabinin and Hostetler, 2016). As a wild-derived species that on average prefers alcohol, prairie voles provide an excellent model to evaluate effects of alcohol in a genetically-heterogeneous population.

An initial study in male and female prairie voles showed that a 24-hour alcohol consumption period via 2-bottle choice procedure during cohabitation did not lead to changes in OXT-ir in the PVN or SON. In contrast, this study observed a significant decrease in the number of OXT-ir fibers in the laterodorsal tegmental area that was independent of sex (Anacker et al., 2014). A seven week-long exposure to a 2-bottle choice procedure in male prairie voles resulted in a significant decrease in OXT-ir in the anterior (but not posterior) PVN (Stevenson et al., 2017b), while a parallel study observed that a shorter period, one week, of alcohol consumption in the 2-bottle choice procedure is sufficient to produce a significant decrease in OXT-ir in PVN, but not SON, of male prairie voles (Walcott and Ryabinin, 2017). A subsequent follow-up study confirmed that one week of alcohol consumption in female prairie voles also results in a decrease in OXT-ir in PVN, but not SON (Walcott and Ryabinin, 2019). Interestingly, though, no colocalization of OXT and the apoptosis marker cleaved caspase 3 was observed in the PVN of alcohol-drinking prairie voles, suggesting that this decrease in the number of PVN OXT neurons is not due to cell death. Overall, significant changes in central OXT levels following alcohol exposure can be observed in males and females, although these changes likely depend on a more complex interplay between paradigm, sex, genotype and species.

3.3. Effects of prenatal alcohol exposure on the OXT system.

Effects of prenatal alcohol exposure on the OXT system demonstrate alcohol’s time-dependent influences on physiological processes. The existence of such effects was hypothesized in an early review (Anderson, 1981) based on previous observations of alcohol’s effects on OXT release in females (described in section 3.A). Gestational exposure to a combination of alcohol and nicotine in Sprague-Dawley rats caused an increase in VTA OXT levels in adolescent male (but not female) offspring, but a decrease in these levels in adulthood (McMurray et al., 2008). A similar exposure to alcohol and nicotine during gestation did not significantly affect OXT mRNA expression in PVN and SON of adult male and female Sprague-Dawley rats, but increased OXTR binding in NAC and hippocampus of adult male (but not female) rats (Williams et al., 2009). Gestational and early postnatal (as a model of human third trimester of pregnancy) alcohol exposure of Long-Evans rats resulted in decreased OXT binding in amygdala of adult rats. Interestingly, the effects appeared to be specific to alcohol in female rats, whereas the effects in males were also attributable to nutritional deficits associated with this method of exposure (Kelly et al., 2009).

The sensitivity of the OXT system to developmental alcohol exposure appears to be evolutionary conserved, as expression of both isotocin (the fish homologue of OXT) and OXTR mRNA are affected by embryonic exposure to alcohol in zebrafish (Coffey et al., 2013; Parker et al., 2014). In fact, a parallel transcriptomic analysis of human embryonic cortex exposed to alcohol ex-vivo and mouse embryonic cortex exposed to alcohol via IP injections in pregnant mice showed an overlap in expression changes in the OXTR- mediated signaling pathway category of genes (Hashimoto-Torii et al., 2011).

Subsequent studies performed in male offspring of Sprague-Dawley rats gestationally exposed to alcohol via a liquid diet observed increases in OXTR binding in infralimbic cortex and CeA during late adolescence. In contrast, during late adolescence, they found no changes in these regions, but did observe increased OXTR binding in the lateral septum (Holman et al., 2018). This group has also detected a decrease in OXT mRNA expression in magnocellular and parvocellular neurons of the PVN and in the SON of male offspring following gestational alcohol exposure, while this change was only detected in the SON of female offspring, and these effects were observed only during late adolescence. Interestingly, limited bedding/nesting material conditions, a model of early life adversity, attenuated the effects of prenatal alcohol exposure in the magnocellular PVN and SON of male offspring (Holman et al., 2021).

Overall, early developmental alcohol exposure can result in transient and/or long-term changes in the central components of the OXT system. These changes are observed during different stages of adolescence and adulthood, and in both males and females. Therefore, alcohol exposure can be considered as one of many early life experiences shaping an individual’s OXT system, which can in turn contribute to the development of AUD (Nylander and Roman, 2012).

4. OXT’s effect on AUD-related measures.

4.1. OXT’s effects on tolerance, alcohol-mediated sedation and thermoregulation.

Data from the field’s earlier preclinical studies are often cited as evidence for OXT’s attenuating effects on the development of alcohol tolerance. However, a thorough analysis of this literature leads to a more complex picture. The initial study on OXT and alcohol tolerance using male C57BL/6 mice showed that while repeated injections of AVP (approximately 0.3 mg/kg, SC, daily) potentiated tolerance developed from liquid diet for up to 9 days of drinking, an equivalent treatment with OXT did not affect tolerance as measured by hypothermic and sedative effects of alcohol (Hoffman et al., 1979). A study in male Swiss mice showed that the same dose of OXT (30 minutes before alcohol injection) decreased the expression of tolerance to hypothermic effects of acute IP injection of alcohol that developed following a 3-day exposure to alcohol vapors in the presence of alcohol metabolism inhibitor pyrazole (Rigter et al., 1980). Authors noted, however, that they could not exclude that this result could be due to an acute hypothermic effect of OXT itself.

A subsequent study showed that OXT injection (up to 0.3 mg/kg, SC) does not alter the acute sedative effects of a large dose of alcohol (5 g/kg IP) in male albino CFLP mice (Szabo et al., 1985). The same and several later studies in male CLFP mice have used a 3-day, repeated IP injections of 4 g/kg alcohol paradigm to assess OXT’s effects on tolerance to alcohol-induced hypothermia. Daily doses of approximately 0.025–0.1 mg/kg OXT injected 2 hours prior to alcohol treatment prevented the development of tolerance (repeatedly-injected animals showed hypothermia if treated with OXT, but not in its absence), whereas a single injection of OXT (0.025 mg/kg) did not affect the expression of tolerance (Szabo et al., 1985). Similarly, body temperatures were lower in animals that received 0.05–0.3 mg/kg OXT 2 hours prior to the third, but not first injection of ethanol, providing further evidence that OXT may inhibit tolerance to alcohol’s hypothermic effects [reviewed in (Kovacs et al., 1998)]. Similarly, a C-terminal tripeptide fragment of OXT (SC, 2 hours pre-ethanol, 0.15 mg/kg, but not 1.5 μg or 1.5 mg/kg) attenuated the development of tolerance to ethanol-induced hypothermia, without affecting body temperature at baseline (Szabo et al., 1987a). The researchers then switched to the intracranial route of OXT administration and found that intracerebroventricular (ICV) OXT dose-dependently (0.3 and 3 ng doses only) resulted in lower body temperature than in control animals when measured 2 hours post-infusion on the second baseline day (before injections of ethanol started). Subsequently, the first injection of ethanol (4 g/kg, IP) produced hypothermia regardless of whether animals were previously treated with OXT. However, a dose-dependent effect of OXT on alcohol-induced hypothermia was observed after the second and third ethanol injections (effect only observed with 0.3 and 3 ng OXT) (Szabo et al., 1989).

Another group of researchers investigated whether SC OXT would affect tolerance to ethanol-induced hypothermia in male Wistar rats. Tolerance was developed following five daily 5 g/kg IP ethanol injections. OXT (SC, 0.16–2.4 mg/kg, 10 min prior to each ethanol treatment) did not affect tolerance to hypothermia following repeated alcohol exposure. On the other hand, the 0.8 and 2.4 mg/kg doses of OXT decreased tolerance to the sedative effects of this alcohol dose. Further complicating the interpretation of these data, a single OXT injection (SC, 0.8 and 2.4 mg/kg), but not four such injections, decreased body temperature in alcohol-naïve rats, and a single 0.8 mg/kg OXT dose decreased the hypothermic response to acute alcohol administration (Pucilowski et al., 1985). Together, these early studies strongly suggest that OXT can regulate tolerance. However, this conclusion is complicated by the use of very high alcohol doses and by the effects of OXT itself on body temperature regulation (Harshaw et al., 2018; Xi et al., 2017).

Studies using lower doses of alcohol were able to more cleanly demonstrate that OXT can decrease tolerance. In particular, OXT (approximately 0.18 mg/kg, IP) decreased tolerance to the hypothermic, myorelaxant and sedative effects of 5 daily alcohol injections (2 g/kg, IP, 2 hours post OXT) in male OF-1 mice, without affecting these measures under basal conditions or following acute alcohol exposure (Jodogne et al., 1991). Interestingly, however, a subsequent investigation by this group showed that the observed tolerance to repeated injections of alcohol was conditioned (and not observed if testing occurred in a different environmental context from that in which subjects were treated), and that repeated administration of OXT diminished this conditioned response. Thus, OXT’s ability to attenuate tolerance to repeated injections of lower ethanol doses was, in part, due to its ability to prevent learning of a conditioned compensatory response to alcohol-induced hypothermia (Tirelli et al., 1992).

A more recent study showed that OXT (1 μg, ICV) can attenuate acute sedative and ataxic effects of alcohol (1.5 and 3 g/kg, IP) in male Wistar rats. The authors also showed that OXT attenuated alcohol-induced potentiation of delta subunit-containing GABA_A receptor activity (when transfected into Xenopus oocytes), Since activity of this receptor is linked to alcohol’s ataxic effects, this finding suggested that OXT can attenuate these effects of alcohol by directly acting on this GABA receptor (Bowen et al., 2015).

Taken together, while administration of OXT can produce an observed attenuation of alcohol tolerance following repeated exposure, it is difficult to disentangle it from OXT’s effects and from ethanol’s acute effects on hypothermia, sedation and development of conditioned reflexes. An additional concern with these studies is that they all have been performed in male mice and rats, with the generalizability of these effects to females yet to be determined.

OXT’s effects on other alcohol-related measures have been investigated in prairie voles and in humans. Specifically, acute OXT (IP, averaged across 1, 3 and 10 mg/kg; effects of individual doses not reported) treatment did not modulate locomotor activity following a single administration of ethanol (2 g/kg, IP) in male and female prairie voles (Stevenson et al., 2017a). In human studies, OXT (IN, approximately 0.8 μg/kg) given to male heavy social drinkers (a total of 32 subjects), increased the accuracy of distinguishing interoceptive heartbeat-based cues (Betka et al., 2018), which could affect acute responses to alcohol. On the other hand, in young human heavy-social drinkers (total of 19 males and 16 females), OXT (IN, approximately 0.8 μg/kg followed by a 0.4 μg/kg booster) did not affect any of an array of measured acute alcohol effects (0.8 g/kg, per os), including subjective mood and drug effect ratings, heart rate and blood pressure, and four behavioral tasks testing cognition, impulsivity and motor dexterity (Vena et al., 2018). Given the general lack of effect of acute OXT treatment on responses to acute alcohol, it should be possible to assess effects of acute or chronic OXT treatment on alcohol tolerance in prairie voles and humans in future studies.

4.2. OXT’s effects on withdrawal-related symptoms

Withdrawal from chronic alcohol produces several symptoms, including general malaise, increased anxiety, hyperalgesia, and even seizures. This section reviews OXT’s effects on these symptoms. Though withdrawal can also increase the motivation to consume alcohol, the effects of OXT on this behavior will be discussed in sections 4.3 and 4.4.

An early study in male C57BL/6 mice did not observe any effects of daily OXT (approximately 0.3 mg/kg, SC) on withdrawal-induced hypothermia or pentylenterazole-precipitated seizures following the cessation of alcohol-supplemented liquid diet (Hoffman et al., 1979). A subsequent study used alternating daily IP injections of tertiary butanol (1.6 g/kg) and ethanol (3 g/kg) in male CFLP albino mice to induce dependence on aliphatic alcohols (treatment for 8 days total). Repeated SC injections of very low doses of OXT (1 μg/kg) 2 hours prior to the administration of alcohols and prior to the administration of picrotoxin (administered to induce seizure) on day 5 increased the incidence of tonic seizures and mortality during precipitated withdrawal, but had no effect in control animals. In contrast, repeated 0.1 mg/kg OXT strongly attenuated the incidence of tonic seizures, blocked myoclonic seizures and decreased mortality during precipitated withdrawal. Notably, these effects were only observed if OXT was repeatedly administered prior to precipitated withdrawal (Szabo et al., 1987b).

A small (total of 9 men and 2 women) but influential randomized double-blind clinical trial demonstrated that IN OXT (approximately 0.5 μg/kg, twice per day for three days) significantly decreased withdrawal symptoms and lowered the threshold for benzodiazepine dose needed to ameliorate these withdrawal symptoms in AUD patients (Pedersen et al., 2013). Another small clinical trial (8 total subjects of both sexes) also showed lower withdrawal scores in patients receiving the same dose of OXT (Pedersen, 2017).

However, a more recent clinical trial with 40 male and female patients following a similarly-scheduled OXT treatment yielded inconclusive results. Specifically, there was a non-statistically significant decrease in benzodiazepine dose required for OXT-treated subjects but no difference in withdrawal scores during detoxification (Melby et al., 2019). These OXT-administered patients did not differ from placebo-administered controls in locomotor activity and sleep patterns (Melby et al., 2020b). However, in a separate study, where patients self-administered OXT (up to 0.5 μg/kg/day) at their own discretion after completing detoxification for 4 weeks, they did report experiencing lower levels of “nervousness” (Melby et al., 2020a).

Overall, the studies on OXT’s effects on symptoms associated with alcohol withdrawal are ambiguous. Since the OXT system is known to regulate nociception and anxiety (Goodin et al., 2015; Neumann and Landgraf, 2012; Neumann and Slattery, 2016; Poisbeau et al., 2018; Steinman et al., 2019), future studies on these measures of alcohol withdrawal could be fruitful.

4.3. OXT’s effects on alcohol consumption

A substantial amount of literature has accumulated in the past 10 years describing OXT’s effects on alcohol intake across different paradigms, ages, species and both sexes (Tables 1 and 2).

4.3.1. OXT’s effects in rats

The first identified study on this topic found that restraint stress- and alcohol deprivation-induced increases in alcohol consumption (2-bottle choice) in alcohol-preferring “iP” rats were attenuated in rats that received 0.3 mg/kg OXT (IP, 30 minutes prior to stress exposure) (Knapp et al., 2010). A study by a different research group using male Wistar rats showed that 10 days of OXT administration (1 mg/kg, IP) during adolescence resulted in increased social exploration and decreased anxiety-like behavior and alcohol intake during 2-bottle choice in adulthood (Bowen et al., 2011). Thus, these studies suggested that OXT administration can prevent the development of heavy drinking at a later timepoint.

A review article by McGregor and Bowen (2012) described an experiment where preference for an alcohol-supplemented sweetener versus 3% sucrose was studied in alcohol-preferring “P” rats using a lickometer system. Remarkably, a single injection of OXT (1 mg/kg, IP) decreased preference towards alcohol-supplemented sweetener solution that lasted for at least 42 days. While this study included male and female rats, vehicle-treated control groups were not included. Further, because the baseline preference was approximately 50%, it is impossible to distinguish whether OXT decreased the rewarding or increased the aversive properties of alcohol (McGregor and Bowen, 2012).

Focusing on potential acute effects of OXT, a separate study in male Sprague-Dawley rats used an unusual 2-hour limited access “drinking-in-the-dark” procedure with three bottles of 0.05% saccharine containing varied concentrations of ethanol (MacFadyen et al., 2016). When the amount of ethanol consumed in this procedure was calculated, OXT (IP, 30 minutes prior to alcohol access) dose-dependently (0.1–0.5 mg/kg, but not 0.05 mg/kg) decreased alcohol consumption compared to baseline, whereas effects on calculated saccharine consumption were inconsistent and did not reach significance. The same study also evaluated effects of OXT on operant self-administration of ethanol-containing gelatin. OXT (0.3 mg/kg, IP, 30 minutes pre-session) significantly decreased self-administration of ethanol-containing, but not plain, gelatin during the 30-minute operant sessions, demonstrating the specificity of OXT’s effects on alcohol intake (MacFadyen et al., 2016).

A study using male Wistar rats demonstrated that OXT (1 μg, ICV) significantly decreased consumption of 20% alcohol and increased avoidance of this solution in a 2-bottle choice procedure 24 hours post-treatment (Peters et al., 2017). The study also demonstrated that this dose of OXT can block alcohol-induced (1.5 g/kg, IP) dopamine release in the nucleus accumbens. Interestingly, though, OXT antagonism did not affect alcohol-induced DA release, suggesting that either exogenous (but not endogenous OXT) can regulate this alcohol-mediated physiological response, or that OXT can exert its effects via OXTR-independent effects (as suggested earlier in results from (Bowen et al., 2015)).

In a subsequent study, ICV administration of OXT (1 μg, immediately prior to test session) also decreased cue-induced reinstatement of operant alcohol self-administration in dependent male Wistar rats, with no effect in non-dependent controls (Hansson et al., 2018), suggesting that OXT could have selective effects on alcohol-seeking in dependent populations.

This idea was further substantiated in a thorough investigation of alcohol self-administration in male Wistar rats following OXT treatment via different ROAs (Tunstall et al., 2019). IP OXT (1 mg/kg) decreased operant alcohol self-administration in dependent and non-dependent rats, whereas 0.25–0.5 mg/kg only decreased this behavior in dependent rats. Similarly, IN OXT (0.25–1 mg/kg) decreased self-administration in dependent rats only. These results were due, at least in part, to OXT-induced decreases in the motivation to consume alcohol, as evidenced by decreased progressive ratio responding following IP (0.125 and 0.25 mg/kg) and IN (1, but not 0.5 mg/kg) OXT in dependent rats. Of note, the 0.25 mg/kg dose of IP OXT decreased locomotor activity and self-administration of other palatable fluids, in contrast to the alcohol-specific effects of IN OXT. Experimenters then used brain-penetrant and non-penetrant OXTR-targeting pharmacology to demonstrate that OXT’s effects on alcohol self-administration in dependent rats are mediated by central OXTR receptors. Further slice physiology experiments showed that OXT (0.5 and 1 μM) decreased evoked GABA transmission in CeA in non-dependent, but not in dependent rats, while decreasing spontaneous GABA responses in both groups. Moreover, OXT (0.5 μM) blocked the faciliatory effects of acute alcohol on GABA release in the CeA of dependent rats, and this effect was specific to action on the OXTR. These data provided strong evidence that OXT selectively affects CeA plasticity with the development of alcohol dependence (Tunstall et al., 2019).

Taken together, these studies found that exogenous OXT can decrease alcohol consumption in male rats. Despite the potential complications of some of the models employed in these studies, they demonstrated that effects of ICV and IN OXT are central and selective to rats that are made dependent on alcohol and involve actions on the extended amygdala. The effect is likely due to decreased motivation to obtain alcohol, but given the low preference for alcohol solutions at baseline, OXT’s effects on the aversive properties of alcohol can’t be excluded. As previously mentioned, a major caveat of these studies in rats (Hansson and Spanagel, 2021) is that all but one involved only male rats, and the effects of OXT on alcohol consumption still need to be thoroughly studied in female rats.

4.3.2. OXT’s effects in mice.

A study in male C57BL/6N mice examined effects of OXT on 2-bottle choice alcohol consumption in two groups: 1) animals that were single-housed for 15 days prior introduction of alcohol, or 2) animals that were housed in chronic subordinate social housing for 15 days prior to switching to single housing and prior to introduction of alcohol (Peters et al., 2013). Single-housed animals had unusually low levels of alcohol intake for C57BL/6 mice, showing no preference for alcohol solutions versus water. However, this low intake was further decreased for 24 hours after a single injection of OXT (10 mg/kg, IP). Mice previously exposed to chronic subordinate housing exhibited higher intake and alcohol preference typical of C57BL/6 mice. However, their intake did not significantly decrease after treatment with this dose of OXT (as measured at 24 hours post-injection). A follow-up experiment performed in a similar manner showed that ICV OXT (0.5 μg) had no effect on alcohol preference in either single-housed mice or mice exposed to the social subordination procedure (Peters et al., 2013). While this result demonstrated fairly limited effects of OXT, the authors only examined intake at 24hr post-treatment and did not assess whether there could have been shorter-lived effects of OXT.

A later study investigated the role of endogenous OXT in regulation of alcohol consumption. Bahi and colleagues (2016) used lentiviruses to overexpress OXTR in the NAC of male C57BL/6J and C57BL/6N mice. This overexpression led to decreased anxiety-like behavior and decreased alcohol intake and preference in a 2-bottle choice procedure, while no effects on taste preference, alcohol-induced sedation, or alcohol metabolism were observed (Bahi et al., 2016). While this strategy is accompanied by expression of OXTR in cells that normally do not express this receptor, these findings strongly implicate endogenous OXT and NAC in regulation of alcohol intake.

A study in male C57BL/6J mice showed that OXT (0.1–10 mg/kg, IP, 30 minutes prior to alcohol introduction) dose-dependently decreased alcohol intake in a 4-hour, 20%-only, “drinking-in-the-dark” procedure as measured by either alcohol drinking, lickometer measures of sipper contacts, or operant self-administration, an effect that was blocked by OXTR antagonism (King et al., 2017). Additionally, the 0.3 mg/kg OXT dose decreased average breakpoint during a progressive ratio schedule of alcohol self-administration. However, the 3 mg/kg dose (but not 0.3 and 1 mg/kg) also had inhibitory effects on locomotor activity, and the 1 mg/kg dose (but not 0.1–0.3 mg/kg) decreased operant self-administration of sucrose (King et al., 2017). Thus, this study provided strong evidence for the effectiveness of low doses of IP OXT to selectively suppress motivation for alcohol during binge-like drinking via action on the OXTR. In a subsequent study by the same group, using both male and female C57BL/6J mice, 1 mg/kg OXT (IP) blocked reinstatement of alcohol self-administration following predator odor exposure or yohimbine administration, as models of stress (King and Becker, 2019). Interestingly, a lower dose of OXT (0.5 mg/kg) blocked reinstatement following predator odor exposure in females, but not in males, suggesting that females could be more sensitive to manipulations of the OXT system than males under these conditions.

Evidence for differential alcohol sensitivity of the OXT system between males and females was also provided by a recent study using OXTR knockout (KO) mice (Rodriguez et al., 2020). Despite being on a C57BL/6J background, wildtype mice in this study showed unusually low levels of alcohol intake in a 2-bottle choice procedure. Female (but not male) mice with a global genetic deletion of OXTR showed higher levels of alcohol intake than wildtype littermate controls. While there was no effect of OXTR deletion on forced swim stress-induced alcohol intake in male or female mice, OXTR KO females (but not males) consumed more saccharine (but not quinine) than wildtype females. Therefore, there is a possibility that genetic manipulation of the OXT system affects intake of different palatable fluids in a sex-specific manner.

A recent study has for the first time evaluated the effects of repeated OXT treatment (3 mg/kg, IP, daily for 4 consecutive days) on established 2-bottle choice alcohol intake in male and female C57BL/6J mice (Caruso et al., 2021). This study, also for the first time in mice, evaluated OXT’s effects on alcohol intake in social settings (half of the animals in the same cage received OXT, and half of the animals received saline), instead of the standard single-housing conditions. OXT selectively decreased alcohol consumption (but not water) in both male and female mice during first 3 hours after administration, and this decrease occurred due to a decrease in number, rather than size, of alcohol drinks. Importantly, though, the effects of repeated OXT administration did fluctuate in amplitude and specificity over time, in that OXT significantly decreased intake on three out of four treatment days, while also producing a non-specific effect (decreasing both alcohol and water consumption) on the fourth day of injection (Caruso et al., 2021).

Overall, studies in mice have shown that OXT inhibits alcohol intake, most likely by decreasing the motivation to consume alcohol in both male and female subjects, and that these effects are mediated by OXTR. Notably, though, it has been demonstrated that OXT’s effects on these behaviors can vary across days of repeated treatment. Additionally, the effects of OXT in mice have yet to be explored in models of heavy alcohol dependence, like they have been in rats. However, just two weeks of alcohol drinking in a 2-bottle choice procedure can lead to hyperalgesia during withdrawal (Smith et al., 2016; Smith et al., 2017), which suggests that the 2-bottle choice procedure in mice can lead to at least mild alcohol dependence.

4.3.3. OXT’s effects in voles.

It is important to test effects of OXT on alcohol-related behaviors in species whose OXT system is adapted to a similar social behavioral repertoire as humans. Acute IP OXT (averaged across 1, 3 and 10 mg/kg, but not analyzed at individual doses) decreased alcohol drinking in both continuous and intermittent 2-bottle choice alcohol drinking procedures in male and female prairie voles (Stevenson et al., 2017a). During intermittent ethanol access, the OXT-induced decrease was maximal for both sexes one hour post-OXT. Interestingly, OXT’s effect did not last more than six hours for females, but was significant at 12 and 24 hours post-treatment for males. During continuous ethanol access, the effect of OXT was observed in both sexes at only one hour post-injection, but this OXT-induced decrease in alcohol consumption was not accompanied by a decrease in alcohol preference. In these experiments, animals were housed in semi-social conditions, i.e. they were housed as same-sex pairs, separated by a mesh-divider. Separating the cage using such dividers allows analysis of drinking in each member of the pair while permitting some degree of olfactory, vocal and visual contact between them. On the other hand, certain studies suggest that the incomplete nature of such contact could serve as an even greater stressor than individual housing (Rettich et al., 2006; Van Loo et al., 2007; Walcott and Ryabinin, 2021). In turn, such stress exposure could influence the effects of OXT on alcohol intake.

A recent study for the first time evaluated effects of repeated OXT on 2-bottle choice alcohol consumption in male and female prairie voles housed in an unconstrained social environment (Walcott and Ryabinin, 2021). OXT (3 mg/kg, IP, daily for four days) significantly decreased alcohol consumption in male and female voles housed together with vehicle-treated, same-sex cagemates. This effect was detectable up to 6 hours post-injection independently of sex. As in the previous vole study, the effect of OXT was not specific to alcohol, as alcohol preference was not affected. The difference between OXT- and vehicle-treated animals was lower on the fourth day of OXT treatment, but this apparent decrease in OXT’s effect was at least in part due to a decrease in alcohol intake in controls. Thus, OXT-treated animals continued to show lower intake on the fourth treatment day versus intake at baseline.

Combined, the studies in prairie voles showed that repeated OXT can have prolonged effects on alcohol consumption in subjects housed in social settings. Similarly to studies in mice, it is important to note that withdrawal from 1 week of alcohol consumption in a 2-bottle choice procedure is accompanied by hyperalgesia (Walcott et al., 2018), indicating that alcohol-consuming voles in these procedures show at least mild alcohol dependence. As a caveat of all rodent studies, there has yet to be an investigation assessing effects of OXT administration for more than 4 days, as this the manner in which OXT would likely be administered in clinical settings.

4.3.4. OXT’s effects in humans.

Studies evaluating its effect on alcohol consumption in humans are only beginning to emerge. A genetic study identified that male Estonian youths homozygous for the AA allele in OXTR polymorphism rs53576 were more frequent alcohol consumers at ages 15 and 18, and were also more likely to have developed AUD at age 25 compared to subjects with other alleles (Vaht et al., 2016). This relationship was not observed in females. Interestingly, the observed effect on AUD could have been in part mediated by social effects of OXT, as male and female subjects with the AA allele who self-reported negative relationships with their teachers (but not their peers) at age 15 were more likely to develop AUD. However, when the same polymorphism was analyzed in adult male Chinese Han subjects, no relationship with alcohol dependence was found. Rather, the GG allele in a separate OXT polymorphism (rs6133010) was significantly associated with alcohol dependence (Yang et al., 2017).

The first small clinical trial (total of 13 men/9 women) in AUD patients performed in the USA showed that IN OXT administration (self-administered, approximately 0.9 μg per administration, three times per day) significantly decreased the number of heavy drinking days and the number of drinks per drinking day during a 12-week treatment period (Pedersen, 2017). A potential attenuating effect of OXT on self-reported anxiety was also observed, but did not reach significance (p=0.098).

On the other hand, a more recent clinical trial (total of 27 men and 11 women) in Norway did not detect a significant effect of OXT (IN, self-administered “when experiencing craving”, up to a maximum of 0.5 μg per day) on the number of days until relapse, the amount of alcohol consumed per day, anxiety, depression, or blood concentration of phosphatydilethanol (a physiological measure of alcohol exposure) during 28 days in alcohol-dependent patients following detoxification (Melby et al., 2020b). As noted in section 4.B, though, OXT-administering patients did score lower on “nervousness” when compared to placebo-administering individuals.

There are obvious differences between the earlier and later clinical trials (different doses and different instructions for use), but it is clear that the ultimate understanding of whether OXT can decrease alcohol consumption in humans should be investigated further. Moreover, given the potential genetic differences in the OXT system that can contribute to differences in alcohol consumption, the effectiveness of OXT could also depend on subject’s genotype, another factor that should be investigated thoroughly in future studies.

4.4. OXT’s effects on motivation to obtain and consume alcohol

4.4.1. Effects of OXT on motivation to obtain alcohol in rodents.

It is important to understand whether OXT administration decreases alcohol consumption by reducing the motivation to consume alcohol (i.e., decreasing spontaneous craving) and responses to alcohol-associated cues (evoked craving). Some evidence for alcohol’s effects on craving have been presented earlier in this review. Thus, OXT decreased progressive ratio responding for alcohol (but not sucrose) in male mice (0.3 mg/kg IP) (King et al., 2017) and male rats (0.125 or 0.25 mg/kg, IP or 1 mg/kg, IN) (Tunstall et al., 2019). OXT has also been shown to decrease cue-primed reinstatement in alcohol-dependent male rats (1 μg ICV) (Hansson et al., 2018) and stress-primed reinstatement in male and female mice (King and Becker, 2019).

The ability of a treatment to affect the motivation to obtain alcohol can be further substantiated using conditioned place preference (CPP). This has been done in two mouse studies using carbetocin. Carbetocin is a more stable analogue of OXT that has prolonged peripheral effects but that differs in receptor specificity from OXT and does not cause receptor internalization after binding (Passoni et al., 2016). Repeated carbetocin treatment (6.4 mg/kg, IP), as well as lentiviral overexpression of OXTR in the NAC of male C57BL/6 mice decreased acquisition, accelerated extinction, and inhibited reinstatement of CPP to alcohol (Bahi, 2015). These experiments strongly suggest that OXTRs in NAC mediate the motivation to seek alcohol and/or sensitivity to alcohol reward.

On the other hand, repeated administration of the same dose of carbetocin in male Swiss mice actually enhanced ethanol-induced CPP (Rae et al., 2018). Thus, rodent studies suggest that while OXT decreases the motivation to obtain alcohol, the effect of its more stable analogue seems to vary across different genotypes. One noticeable caveat is the sparsity of studies that have examined the effects of OXT on the motivation to obtain alcohol in female rodents.

4.4.2. Effects of alcohol on motivation to obtain alcohol in humans.

As described in section 4.B, the first study that evaluated effects of OXT (IN, approximately 0.5 μg/kg, twice per day for three days) on withdrawal symptoms in male and female AUD patients detected significantly lower scores on the Alcohol Craving Visual Analog Scale during one of the three days of admission to the clinic (Pedersen et al., 2013). On the other hand, a later, smaller clinical trial by the same group, mentioned in section 4.C.1, did not detect statistically significant effects of OXT treatment on craving scores, although heavy drinking was attenuated (Pedersen, 2017).

A study in heavy alcohol-using (but not physically dependent) individuals (total of 19 men and 13 women) suggested that OXT’s effects on craving indices could depend on personality traits (Mitchell et al., 2016), in that while OXT-administered (IN, 0.8 μg/kg) subjects generally showed a strong trend towards increased cue-induced craving (Alcohol Urge Questionnaire), this reaction depended on social attachment style. More specifically, OXT reduced craving in individuals anxious about their social attachment, but enhanced craving in individuals with low attachment anxiety. OXT also significantly decreased approach bias towards appetitive stimuli (not specific to alcohol) in an approach/avoidance task.

Hansson et al (2018) also studied OXT’s effect on cue reactivity in male heavy alcohol-consuming volunteers (a total of 12 subjects). OXT (IN, 0.5 μg/kg) decreased neural responses to an alcohol cue in the insular cortex, hippocampal/parahippocampal formation, cingulate cortex, the inferior and the medial frontal cortex and in the visual and motor regions, as measured by fMRI (Hansson et al., 2018). The same dose of OXT in 13 heavy-drinking males also decreased connectivity between NAC and the cuneus and between thalamus and cortex, but enhanced connectivity between the paracingulate and precentral gyri (Bach et al., 2019). Interestingly, NAC-cuneus connectivity negatively correlated with subjective cue-induced craving, and authors interpreted these changes as an OXT-induced attenuation of alcohol cue saliency and improved inhibitory control over craving. These latter two studies, however, did not report whether OXT affected the subjective rating of craving.

A substantially larger study (67 participants) in male veterans with comorbid posttraumatic stress disorder (PTSD) and AUD, however, did not detect a significant effect of OXT (IN, 0.8 μg/kg) on social stress-induced craving, even though OXT administration marginally reduced the stress-induced cortisol response (Flanagan et al., 2019). Similarly, a study with 47 male PTSD/AUD comorbid participants did not find any effects of OXT (IN, 0.4 or 0.8 μg/kg) on alcohol cue-induced subjective craving or on alcohol cue-induced changes in heart rate (Stauffer et al., 2019), suggesting that OXT may not be as effective in decreasing alcohol craving in this patient population.

In addition, a study described in section 4.C.4 did not identify any effects of OXT (IN, “when experiencing craving”, up to a maximum of 0.5 μg per day) on alcohol craving (Alcohol Craving Questionnaire) following 28 days of treatment in male and female alcohol-dependent patients following detoxification (Melby et al., 2020b).

Taken together, while initial studies in male rodents and humans suggested that OXT administration could directly affect the motivation to obtain alcohol, more recent investigations contradict this idea. Thus, OXT could be regulating alcohol consumption through mechanisms aside from those involved in craving, for instance, those involved in social behaviors.

4.5. OXT’s effects on social aspects of AUD

OXT is involved in the regulation of social behaviors (Carter et al., 2020; Insel et al., 1995; McGregor and Bowen, 2012; Meyer-Lindenberg et al., 2011), and alcohol is considered a “social” drug (capable of both promoting and inhibiting social attachments). However, effects of OXT on alcohol-induced social behaviors have rarely been investigated.

The earliest series of studies in this direction identified a relationship between polymorphisms in the OXTR gene and alcohol-related aggression. The OXTR polymorphism rs4564970 was associated with the effects of acute alcohol intake (0.7 g/kg per os) on aggression in 116 Finnish men (Johansson et al., 2012a). Thereafter, it was reported that the same OXTR polymorphism moderated self-reported effects of alcohol consumption on aggressive behavior, and that OXTR polymorphisms rs4564970 and rs1488467 moderated self-reported effects of alcohol consumption on anger in male and female Finnish participants (1498 men and 2079 women) (Johansson et al., 2012b). Subsequent rigorous analysis of interactions between 33 polymorphisms in or around the OXTR gene in two independent experiments in Finnish male university students (116 and 119 men in each) confirmed that differences in OXTR genotypes were also associated with increased aggression in a laboratory test following alcohol consumption (0.7 or 1 g/kg, per os) (LoParo et al., 2016).

Despite the findings outlined above, OXT is better known for its involvement in affiliative behavior than in aggression. Association of OXTR in alcohol-related affiliation was studied in female mice on C57BL/6J background using ethanol-conditioned social preference (Wood et al., 2015). With repeated exposures, female WT mice developed a preference for a mouse to which they were exposed after receiving an ethanol injection (2 g/kg, IP) versus a mouse to which they were exposed after receiving a saline injection. In contrast, ethanol-injected female OXTR KO mice did not develop such preference. Importantly, this effect of genotype on alcohol-related affiliation occurred despite no differences in alcohol-induced olfactory plasticity or sedation in female OXTR KO versus WT mice.

The OXT system has also been investigated for its role in other social phenomena following alcohol exposure. For example, OXTR antagonism did not affect enhancement of vicarious fear conditioning following an acute ethanol injection (1.5 g/kg, IP) in male C57BL/6J mice, suggesting that OXT does not play a role in alcohol’s effect on this type of empathy-like behavior (Sakaguchi et al., 2018). On the other hand, OXT (IN, 0.4 μg/kg, and to a lesser degree 0.8 μg/kg) increased scores of automatic imitation in 43 healthy control men, but not in 54 men with comorbid PTSD/AUD, indicating that OXT can contribute to alcohol’s effect on a different type of empathic behavior (Morrison et al., 2020).

An fMRI study showed that OXT administration (IN, 0.5 μg/kg) decreased activation of bilateral amygdala, hippocampus and parts of occipital gyrus induced by exposure to emotional faces in 13 male heavy social drinkers (Bach et al., 2020). Interestingly, OXT-mediated attenuation of the amygdala response was associated with lower subjective alcohol craving and lower percentage of heavy-drinking days preceding the test in these subjects. This finding suggests that an individual’s history of alcohol consumption affects his perception of others following OXT treatment.

Taken together, these experiments suggest that the OXT system can modulate alcohol’s effects on a variety of social interactions. In turn, social interactions could affect the selection of conditions of social interventions and support. Indeed, a combined low risk score of OXTR alleles associated with “negative social behavior and related interpersonal characteristics” was linked to affiliating with friends who had less substance use in a substance use intervention context (but not in the control condition) in 1418 adolescents of both sexes attending 9^th grade (Cleveland et al., 2018). It has also been shown that in 269 male and female alcohol-dependent adults, OXT polymorphisms moderated the negative relationship between social support and psychiatric distress (Love et al., 2018). The OXT system, therefore, could affect treatment outcomes by affecting the choice of group affiliations in individuals with AUD.

5. Conclusions and future directions.

The above review of existing work on the interactions between alcohol and OXT identifies several substantial gaps in the literature that could affect finding a better strategy for targeting the OXT system in AUD. To improve such strategies, we need to consider mechanisms by which OXT could be affecting AUD symptoms.

5.1. Do deficits or other changes in the OXT system exist across the majority of AUD patients, and does exogenous OXT compensate for such changes?

The answer for the first part of this question is unequivocal: yes, acute alcohol does produce effects in the peripheral system (i.e. inhibits stimulus-evoked OXT release) across both male and female subjects, and chronic alcohol affects central OXT and OXTR levels and corresponding neuroanatomical structures across males and females. There are indeed more reports on alcohol’s long-term effects in male rats than in female rats and in male human post-mortem brains than in female postmortem brains, though changes in both mice and prairie voles of both sexes have been demonstrated. However, the literature clearly identifies that the consequences of alcohol exposure on male and female brains change dynamically overtime. Alcohol causes changes in OXT release and central OXT/OXTR expression that can emerge or dissipate depending on the subject’s genotype, age, sex, and environmental circumstances. These observations indicate that there are factors that either inhibit or facilitate the initial effects of alcohol on the OXT system. If OXT treatment is acting to compensate for these initial effects, it could function to counteract the systemic alterations which developed with repeated alcohol exposure. It also needs to be kept in mind that an AUD medication can act independently of alcohol’s effects on the OXT system to produce beneficial effects.

5.2. We need to consider several possibilities of behavioral mechanisms of OXT’s effects as a treatment of AUD. Is OXT acting by decreasing craving for alcohol?

One early clinical observation in humans, as well as several rodent CPP and progressive ratio operant reinforcement studies suggest that this is a possibility. However, more recent clinical studies don’t confirm these effects, and certain CPP studies suggest that OXT may actually potentiate alcohol craving or sensitivity to alcohol reward. While more studies testing potential effects of OXT on craving are needed, we also need to consider other possibilities.

5.3. Is OXT acting to decrease tolerance to the effects of repeated alcohol exposure?

It can be theorized that decreased tolerance to aversive effects of alcohol could either decrease or increase the consumption of alcohol. However, the absence of data in female animals, the use of forced methods of alcohol administration, the use of mostly short-term repeated treatments, the lack of effects in early studies with longer treatments, and the intermittent observations of effects on acute responses to alcohol across studies, severely complicate the picture. This direction of research was extensive in the 1980s, and comprehensive investigations of OXT’s effects on tolerance using more sophisticated tools, and in male and female subjects, are needed.

5.4. Is OXT modulating alcohol-induced anxiolysis?

Alcohol has anxiolytic properties, and increased anxiety often accompanies alcohol dependence. When reviewed, OXT is often described as part of an anti-anxiety or “anti-stress” mechanism, so it is logical to propose that OXT decreases alcohol consumption by decreasing dependence-associated stress levels. A few studies in rodents have observed decreased anxiety associated with decreased alcohol consumption following OXT administration. OXT also decreased stress-induced reinstatement of alcohol-administration in mice. However, effects of OXT on alcohol administration in rodents have also been observed under standard basal alcohol consumption conditions when increased anxiety is not expected, suggesting that OXT can act on alcohol drinking without affecting anxiety-like behaviors. Moreover, clinical studies that did detect an effect of OXT on anxiety or stress responses, did not detect effects on craving. Again, more research on this subject is needed and other possibilities should be considered.

5.5. Is OXT affecting alcohol consumption by affecting severity of withdrawal?

Early clinical studies suggested exactly this possibility. However, more recent clinical trials contradict this observation. (Though, the differences in outcome could be due to differences in experimental design). Outside of two contradictory early mouse studies (with one attenuating and another demonstrating no effect), the absence of studies on OXT’s ability to modulate withdrawal symptoms is glaring. Comprehensive studies using appropriate models of alcohol self-administration, and perhaps assessing phenotypes that are more relevant to humans (for example, withdrawal-induced hyperalgesia or withdrawal-induced anxiety), are very much needed.

5.6. Is OXT affecting alcohol consumption by altering social behaviors?

One possibility is that alcohol produces rewarding effects by hijacking social reward neurocircuitry, which is sensitive to OXT. While intriguing, this theory suggests that OXT would still affect craving for alcohol. As mentioned above, this mechanism is plausible, but is currently subject to contradictory results. It is possible that these contradictory results are due to social contexts in which craving is measured across studies. However, OXT may be affecting social interactions and affiliations, and thereby, indirectly, distracting from drinking behaviors and habits to improve treatment outcome. While rodent literature on this topic is almost non-existent, several recent clinical studies clearly suggest this possibility. Please also note that although alcohol researchers using non-human primates (NHP) have shown that exogenous OXT can cross the blood brain barrier and affect behavior (Jiang and Platt, 2018; Lee et al., 2020; Parr et al., 2013), studies on the effects of OXT on AUD-related symptoms in NHP have yet to be conducted. Such studies could bridge the gap between rodent and clinical studies. Importantly, with the increased recent capabilities in the social neurobiology field, the absence of research on OXT’s contribution to social aspects of AUD calls for clever experimental design and conclusive answers.

Highlights.

• Alcohol inhibits oxytocin release from hypophyseal terminals in males and females.

• Alcohol affects centers of oxytocin production in males and females.

• Alcohol’s effects on the oxytocin system can change with repeated exposure.

• Oxytocin administration tends to decrease alcohol consumption in males and females.

• Oxytocin’s effects on tolerance, withdrawal, social behaviors need further studies.