The Leu72Met Polymorphism of the Prepro-ghrelin Gene is Associated With Alcohol Consumption and Subjective Responses to Alcohol: Preliminary Findings

Animal studies reveal that the ghrelin pathway regulates alcohol-mediated responses

Alcohol use disorder (AUD) is often described as repeated phases of binge drinking, compulsive alcohol-taking, craving for alcohol during withdrawal, and drinking with an aim to a reduce the negative consequences. Although multifaceted, alcohol-induced reward is one aspect influencing the former three of these. The neurobiological mechanisms regulating AUD processes are complex and one of these systems is the gut-brain peptide ghrelin. The vast physiological properties of ghrelin are mediated via growth hormone secretagogue receptor (GHSR, ghrelin receptor). Ghrelin is well known for its ability to control feeding, hunger, and metabolism. Moreover, ghrelin signaling appears central for alcohol-mediated responses; findings reviewed herein. In male rodents GHSR antagonism reduces alcohol consumption, prevents relapse drinking, and attenuates the motivation to consume alcohol. On the other hand, ghrelin increases the consumption of alcohol. This ghrelin-alcohol interaction is also verified to some extent in humans with high alcohol consumption. In addition, either pharmacological or genetic suppression of GHSR decreases several alcohol-related effects (behavioral or neurochemical). Indeed, this suppression blocks the alcohol-induced hyperlocomotion and dopamine release in nucleus accumbens as well as ablates the alcohol reward in the conditioned place preference model. Although not fully elucidated, this interaction appears to involve areas central for reward, such as the ventral tegmental area (VTA) and brain nodes targeted by VTA projections. As reviewed briefly, the ghrelin pathway does not only modulate alcohol-mediated effects, it regulates reward-related behaviors induced by addictive drugs. Although personality traits like impulsivity and risk-taking behaviors are common in patients with AUD, the role of the ghrelin pathway thereof is unknown and remains to be studied. In summary, the ghrelin pathway regulates addiction processes like AUD and therefore the possibility that GHSR antagonism reduces alcohol or drug-taking should be explored in randomized clinical trials.

Role of the ghrelin system in alcohol use disorder and alcohol-associated liver disease: A narrative review

Unhealthy alcohol consumption is a global health problem. Adverse individual, public health, and socioeconomic consequences are attributable to harmful alcohol use. Epidemiological studies have shown that alcohol use disorder (AUD) and alcohol-associated liver disease (ALD) are the top two pathologies among alcohol-related diseases. Consistent with the major role that the liver plays in alcohol metabolism, uncontrolled drinking may cause significant damage to the liver. This damage is initiated by excessive fat accumulation in the liver, which can further progress to advanced liver disease. The only effective therapeutic strategies currently available for ALD are alcohol abstinence or liver transplantation. Any molecule with dual-pronged effects at the central and peripheral organs controlling addictive behaviors and associated metabolic pathways are a potentially important therapeutic target for treating AUD and ALD. Ghrelin, a hormone primarily derived from the stomach, has such properties, and regulates both behavioral and metabolic functions. In this review, we highlight recent advances in understanding the peripheral and central functions of the ghrelin system and its role in AUD and ALD pathogenesis. We first discuss the correlation between blood ghrelin concentrations and alcohol use or abstinence. Next, we discuss the role of ghrelin in alcohol-seeking behaviors and finally its role in the development of fatty liver by metabolic regulations and organ crosstalk. We propose that a better understanding of the ghrelin system could open an innovative avenue for improved treatments for AUD and associated medical consequences, including ALD.

Role of Leu72Met of GHRL and Gln223Arg of LEPR Variants on Food Intake, Subjective Appetite, and Hunger-Satiety Hormones

Appetite regulation has been recognized as a promising target for the prevention of obesity, which has become a worldwide health issue. Polymorphisms in the genes of hormones or receptors including Leu72Met for ghrelin and Gln223Arg for the leptin receptor could play a role in dietary intake, hunger, and satiety process. The aim of this study was to analyze subjective appetite assessments, dietary intake, and appetite hormones in relationship to these polymorphisms. Subjects (n = 132) with normal BMIs were enrolled. Dietary intake was analyzed with 3-day diet records. Subjective appetite was measured by visual analogue scales. Biochemical parameters were measured after 12 h of fasting and 120′ following ingestion of a test meal. Ghrelin and leptin levels were measured by ELISA assay (enzyme-linked immunosorbent assay) and insulin by chemiluminescence assay. The polymorphisms were determined by allelic discrimination using TaqMan^® probes. Fasting ghrelin levels differed significantly between men and women. The consumption of fruit and bread/starch with added sugar servings, as indicated by dietary records, and measured ghrelin levels were higher in carriers of Leu72Met/Met72Met compared to Leu72Leu carriers; total sugar intake was higher in Gln223Gln carriers than in Gln223Arg/Arg223Arg carriers. In conclusion, the Leu72Met and Gln223Arg polymorphism in ghrelin and LEPR may contribute to differential responses to a standardized meal as evidenced by higher postprandial levels of ghrelin and may also contribute to a higher dietary sugar intake.

Therapeutic potential of GHSR-1A antagonism in alcohol dependence, a review

Growth hormone secretagogue receptor type 1A (GHSR-1A) is a functional receptor of orexigenic peptide ghrelin and is highly expressed in mesolimbic dopaminergic systems that regulate incentive value of artificial reward in substance abuse. Interestingly, GHSR-1A has also shown ligand-independent constitutive activity. Alcohol use disorder (AUD) is one of the growing concerns worldwide as it involves complex neuro-psycho-endocrinological interactions. Positive correlation of acylated ghrelin and alcohol-induced human brain response in the right and left ventral striatum are evident. In the last decade, the beneficial effects of ghrelin receptor (GHSR-1A) antagonism to suppress artificial reward circuitries and induce self-control for alcohol consumption have drawn significant attention from researchers. In this updated review, we summarize the available recent preclinical, clinical, and experimental data to discuss functional, molecular actions of central ghrelin-GHSR-1A signaling in different craving levels for alcohol as well as to promote "GHSR-1A antagonism" as one of the potential therapies in early abstinence.

Insight into the role of the gut-brain axis in alcohol-related responses: Emphasis on GLP-1, amylin, and ghrelin

Alcohol use disorder (AUD) contributes substantially to global morbidity and mortality. Given the heterogenicity of this brain disease, available pharmacological treatments only display efficacy in sub-set of individuals. The need for additional treatment options is thus substantial and is the goal of preclinical studies unraveling neurobiological mechanisms underlying AUD. Although these neurobiological processes are complex and numerous, one system gaining recent attention is the gut-brain axis. Peptides of the gut-brain axis include anorexigenic peptide like glucagon-like peptide-1 (GLP-1) and amylin as well as the orexigenic peptide ghrelin. In animal models, agonists of the GLP-1 or amylin receptor and ghrelin receptor (GHSR) antagonists reduce alcohol drinking, relapse drinking, and alcohol-seeking. Moreover, these three gut-brain peptides modulate alcohol-related responses (behavioral and neurochemical) in rodents, suggesting that the alcohol reduction may involve a suppression of alcohol's rewarding properties. Brain areas participating in the ability of these gut-brain peptides to reduce alcohol-mediated behaviors/neurochemistry involve those important for reward. Human studies support these preclinical studies as polymorphisms of the genes encoding for GLP-1 receptor or the ghrelin pathway are associated with AUD. Moreover, a GLP-1 receptor agonist decreases alcohol drinking in overweight patients with AUD and an inverse GHSR agonist reduces alcohol craving. Although preclinical and clinical studies reveal an interaction between the gut-brain axis and AUD, additional studies should explore this in more detail.

Dual Effect of the GHRL Gene Variant in the Molecular Pathogenesis of Obesity

Obesity is as a global health problem due to its interaction with complex chronic disorders such as cardiovascular disorders, type 2 diabetes mellitus (T2DM) and cancer. Despite the fact that pathogenesis of obesity is not yet clearly understood, it is associated with a combination of psychological, environmental and various genetic factors. Here, employing a case-control design, we aimed to examine the effects of the GHRL c.152C>T (p.Arg51Gln) (rs34911341) and c.214G>T (p.Leu72Met) (rs696217) markers on susceptibility to obesity in a Turkish-Cypriot population, as well as to evaluate whether these markers affect biochemical parameters and show their putative functional consequences. This study involved 211 Turkish-Cypriot subjects (106 obese and 95 non obese). Genotyping for the GHRL gene polymorphisms was performed by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis. Our results indicate that the GHRL Leu72Met polymorphism was found to be significantly higher in obese patients, with respect to genotypic (p = 0.0012) and allelic (p = 0.0005) frequencies. Strikingly, the rs696217 GT genotype (heterozygous) had significantly lower serum high-density lipoprotein cholesterol (HDL-C) (p = 0.015) than GG (wild type) genotypes. Overall, Leu72Met susceptibility variant may be considered as risk and crucial marker for both obesity and cholesterol metabolism in the community of Turkish-Cypriots. Thus, the dual effect of the GHRL gene Leu72Met variant may be used for clinical diagnosis.

Biochemical Mechanisms Associating Alcohol Use Disorders with Cancers

Simple Summary

Of all yearly deaths attributable to alcohol consumption globally, approximately 12% are due to cancers, representing approximately 0.4 million deceased individuals. Ethanol metabolism disturbs cell biochemistry by targeting the structure and function of essential biomolecules (proteins, nucleic acids, and lipids) and by provoking alterations in cell programming that lead to cancer development and cancer malignancy. A better understanding of the metabolic and cell signaling realm affected by ethanol is paramount to designing effective treatments and preventive actions tailored to specific neoplasias.

Abstract

The World Health Organization identifies alcohol as a cause of several neoplasias of the oropharynx cavity, esophagus, gastrointestinal tract, larynx, liver, or female breast. We review ethanol’s nonoxidative and oxidative metabolism and one-carbon metabolism that encompasses both redox and transfer reactions that influence crucial cell proliferation machinery. Ethanol favors the uncontrolled production and action of free radicals, which interfere with the maintenance of essential cellular functions. We focus on the generation of protein, DNA, and lipid adducts that interfere with the cellular processes related to growth and differentiation. Ethanol’s effects on stem cells, which are responsible for building and repairing tissues, are reviewed. Cancer stem cells (CSCs) of different origins suffer disturbances related to the expression of cell surface markers, enzymes, and transcription factors after ethanol exposure with the consequent dysregulation of mechanisms related to cancer metastasis or resistance to treatments. Our analysis aims to underline and discuss potential targets that show more sensitivity to ethanol’s action and identify specific metabolic routes and metabolic realms that may be corrected to recover metabolic homeostasis after pharmacological intervention. Specifically, research should pay attention to re-establishing metabolic fluxes by fine-tuning the functioning of specific pathways related to one-carbon metabolism and antioxidant processes.

Ghrelin modulates dopaminergic neuron formation and attention deficit hyperactivity disorder-like behaviors: From animals to human models

Attention deficit hyperactivity disorder (ADHD) is one of the most prevalent psychiatric disorders in children. The orexigenic hormone ghrelin is important in neuroprotection and neurodevelopment, which may play an important role in psychopathogenesis of ADHD. This study aimed to systematically investigate the genomic and pharmacological manipulations of ghrelin functioning in ADHD-like symptoms in zebrafish models and validated the effects of ghrelin polymorphisms in human subjects with ADHD. We firstly generated ghrelin^Δ/Δ zebrafish mutant, which displayed hyperactive, attention deficit-like and impulsive-like behaviors, as well as endophenotypes, mimicking human ADHD. Ghrelin^Δ/Δ zebrafish exhibited downregulated expression levels of wnt1, wnt3a, wnt5a that are critical for dopaminergic neuron development to possibly regulate their number and spatial organization. Pharmacological blockade of wnt signaling with XAV939 induced a reduced moving activity and less dopaminergic neurons; whereas, wnt agonist SB415286 rescued hyperactivity and dopaminergic neuron loss in ghrelin^Δ/Δ zebrafish. In addition, we further identified and validated a SNP, rs696217, on orexigenic hormone preproghrelin/ghrelin (T408T, Met72Met) to be associated with a higher risk of ADHD in a case-controlled association study with 248 subjects with ADHD and 208 subjects of healthy controls. Together, our results reveal a novel endogenous role for orexigenic hormone ghrelin in ADHD, which provides insights into genetic regulation and drug screens for the identification of novel treatments of ADHD.

THE INTRIGUING LIGAND-DEPENDENT AND LIGAND-INDEPENDENT ACTIONS OF THE GROWTH HORMONE SECRETAGOGUE RECEPTOR ON REWARD-RELATED BEHAVIORS

The growth hormone secretagogue receptor (GHSR) is a G-protein-coupled receptor (GPCR) highly expressed in the brain, and also in some peripheral tissues. GHSR activity is evoked by the stomach-derived peptide hormone ghrelin and abrogated by the intestine-derived liver-expressed antimicrobial peptide 2 (LEAP2). In vitro, GHSR displays ligand-independent actions, including a high constitutive activity and an allosteric modulation of other GPCRs. Beyond its neuroendocrine and metabolic effects, cumulative evidence shows that GHSR regulates the activity of the mesocorticolimbic pathway and modulates complex reward-related behaviors towards different stimuli. Here, we review current evidence indicating that ligand-dependent and ligand-independent actions of GHSR enhance reward-related behaviors towards appetitive stimuli and drugs of abuse. We discuss putative neuronal networks and molecular mechanisms that GHSR would engage to modulate such reward-related behaviors. Finally, we briefly discuss imaging studies showing that ghrelin would also regulate reward processing in humans. Overall, we conclude that GHSR is a key regulator of the mesocorticolimbic pathway that influences its activity and, consequently, modulates reward-related behaviors via ligand-dependent and ligand-independent actions.

To Infuse or Ingest in Human Laboratory Alcohol Research

Human alcohol laboratory studies use two routes of alcohol administration: ingestion and infusion. The goal of this paper was to compare and contrast these alcohol administration methods. The work summarized in this report was the basis of a 2019 Research Society on Alcoholism Roundtable, "To Ingest or Infuse: A Comparison of Oral and Intravenous Alcohol Administration Methods for Human Alcohol Laboratory Designs." We review the methodological approaches of each and highlight strengths and weaknesses pertaining to different research questions. We summarize methodological considerations to aid researchers in choosing the most appropriate method for their inquiry, considering exposure variability, alcohol expectancy effects, safety, bandwidth, technical skills, documentation of alcohol exposure, experimental variety, ecological validity, and cost. Ingestion of alcohol remains a common and often a preferable, methodological practice in alcohol research. Nonetheless, the main problem with ingestion is that even the most careful calculation of dose and control of dosing procedures yields substantial and uncontrollable variability in the participants' brain exposures to alcohol. Infusion methodologies provide precise exposure control but are technically complex and may be limited in ecological validity. We suggest that alcohol ingestion research may not be the same thing as alcohol exposure research; investigators should be aware of the advantages and disadvantages that the choice between ingestion and infusion of alcohol invokes.

GPCR and Alcohol-Related Behaviors in Genetically Modified Mice

G protein-coupled receptors (GPCRs) constitute the largest class of cell surface signaling receptors and regulate major neurobiological processes. Accordingly, GPCRs represent primary targets for the treatment of brain disorders. Several human genetic polymorphisms affecting GPCRs have been associated to different components of alcohol use disorder (AUD). Moreover, GPCRs have been reported to contribute to several features of alcohol-related behaviors in animal models. Besides traditional pharmacological tools, genetic-based approaches mostly aimed at deleting GPCR genes provided substantial information on how key GPCRs drive alcohol-related behaviors. In this review, we summarize the alcohol phenotypes that ensue from genetic manipulation, in particular gene deletion, of key GPCRs in rodents. We focused on GPCRs that belong to fundamental neuronal systems that have been shown as potential targets for the development of AUD treatment. Data are reviewed with particular emphasis on alcohol reward, seeking, and consumption which are behaviors that capture essential aspects of AUD. Literature survey indicates that in most cases, there is still a gap in defining the intracellular transducers and the functional crosstalk of GPCRs as well as the neuronal populations in which their signaling regulates alcohol actions. Further, the implication of only a few orphan GPCRs has been so far investigated in animal models. Combining advanced pharmacological technologies with more specific genetically modified animals and behavioral preclinical models is likely necessary to deepen our understanding in how GPCR signaling contributes to AUD and for drug discovery.

Ghrelin and aggressive behaviours-Evidence from preclinical and human genetic studies

Aggressive behaviour is of crucial importance in the defence for limited resources including food and mates and involves central serotonin as well as dopamine signalling. As ghrelin modulates food intake and sexual behaviour we initially investigated the hypothesis that central ghrelin signalling regulates aggressive behaviour in the resident intruder paradigm in male mice. Moreover, interaction between ghrelin signalling and serotonergic, noradrenergic as well as dopaminergic neurotransmission in aggression was investigated. The relevance of ghrelin for human aggression per se as well as for aggression induced by alcohol was evaluated in a human genetic association study comprising young men (n = 784) from the normal population assessed for anti-social behaviours. The present study demonstrates that central ghrelin infusion, but not ghrelin administered systemically, increases aggression. Moreover aggressive behaviour is decreased by pharmacological suppression of the growth hormone secretagogue receptor-1 A (GHSR-1A) by JMV2959. As indicated by the ex vivo biochemical data serotonin, rather than dopamine or noradrenaline, in amygdala may have central roles for the ability of JMV2959 to reduce aggression. This link between central serotonin, GHSR-1A and aggression is further substantiated by the behavioural data showing that JMV2959 cannot decrease aggression following depletion of central serotonin signalling. The genetic association study demonstrates that males carrying the Leu72Leu genotype of the pre-pro-ghrelin gene and displaying hazardous alcohol use are more aggressive when compared to the group carrying the Met-allele. Collectively, this contributes to the identification of central ghrelin pathway as an important modulator in the onset of aggressive behaviours in male mice.

The preliminary investigation of orexigenic hormone gene polymorphisms on posttraumatic stress disorder symptoms

Orexigenic hormones are a group of hormones that can up-regulate appetite. Current studies have shown that orexigenic hormones also play important roles in stress responses and may be implicated in regulation of fear memory. However, these conclusions lack evidence from human studies. In this study, we examined associations between orexigenic hormone genes and fear-related mental disorders by investigating main, G × E, and G × G effects of ghrelin and orexin gene single nucleotide polymorphisms (SNPs) on human posttraumatic stress disorder (PTSD) symptoms in 1134 Chinese earthquake survivors. SNPs Leu72Met of the GHRL gene (rs696217), Ile408Val of the HCRTR1 gene (rs2271933) and Val308Ile of the HCRTR2 gene (rs2653349) were genotyped. None of the SNPs showed significant main or G × E effects. However, a significant interaction effect between GHRL rs696217 and HCRTR1 rs2271933 was found to predict the PTSD Checklist (PCL-5) total score (P =  0.007). Further analysis revealed different interaction patterns in males and females. For females, the rs2271933 G allele was associated with an increased PCL-5 total score (B = 2.59, P =  0.024) when the rs696217 genotype TT/TG was present. For males, the rs696217 T allele is associated with an increased PCL-5 total score (B = 3.62, P =  0.040) when the rs2271933 genotype GG/GA was present. These current findings expand our knowledge of physiological function of the orexigenic hormone system, and suggest its involvement in development of fear-related mental disorders such as PTSD.

Gut-brain axis and addictive disorders: A review with focus on alcohol and drugs of abuse

Due to the limited efficacy of existing medications for addictive disorders including alcohol use disorder (AUD), the need for additional medications is substantial. Potential new medications for addiction can be identified through investigation of the neurochemical substrates mediating the ability of drugs of abuse such as alcohol to activate the mesolimbic dopamine system. Interestingly, recent studies implicate neuropeptides of the gut-brain axis as modulators of reward and addiction processes. The present review therefore summarizes the current studies investigating the ability of the gut-brain peptides ghrelin, glucagon-like peptide-1 (GLP-1), amylin and neuromedin U (NMU) to modulate alcohol- and drug-related behaviors in rodents and humans. Extensive literature demonstrates that ghrelin, the only known orexigenic neuropeptide to date, enhances reward as well as the intake of alcohol, and other drugs of abuse, while ghrelin receptor antagonism has the opposite effects. On the other hand, the anorexigenic peptides GLP-1, amylin and NMU independently inhibits reward from alcohol and drugs of abuse in rodents. Collectively, these rodent and human studies imply that central ghrelin, GLP-1, amylin and NMU signaling may contribute to addiction processes. Therefore, the need for randomized clinical trials investigating the effects of agents targeting these aforementioned systems on drug/alcohol use is substantial.

Alcohol consumption is positively associated with fasting serum ghrelin in non-dependent adults: Results from the population-based LIFE-Adult-Study

BACKGROUND

Animal experiments and studies in alcohol dependent patients indicate that ghrelin signaling in the brain is causally involved in the regulation of alcohol reward and intake. Increasing ghrelin levels enhances alcohol craving and intake, blocking ghrelin receptors abolishes these effects. If ghrelin is also involved in non-dependent alcohol consumption in humans, though, remains unknown. The aim was therefore to investigate the relationship between ghrelin serum levels and alcohol consumption in a large population-based sample.

METHODS

Total ghrelin was determined after an overnight fast in 1666 subjects participating in a population-based cross-sectional study ('LIFE') including 10,000 adults. 1521 subjects were included in this analysis. Alcohol consumption was assessed using a food frequency questionnaire (FFQ). Multiple linear regression analyses and extreme group comparisons testing for statistical differences of alcohol consumption between the highest and lowest quartile according to ghrelin levels were performed.

RESULTS

Alcohol consumption was positively associated with serum ghrelin; total sample: β = 0.003, p = 0.002; men: β = 0.005, p = 0.023; women: β = 0.002, p = 0.007, adjusted for age, BMI and smoking status. Mean alcohol consumption in men/women belonging to the highest quartile of serum ghrelin levels (men: 21.5 (21.1) g/day; women: 7.5 (11.4) g/day) was considerably higher than in those belonging to the lowest quartile (men: 16.5 (19.3) g/day p < 0.002; women: 4.59 (10.7) g/day p = 0.0001).

CONCLUSION

This is the first study showing that alcohol consumption is positively associated with serum ghrelin in a population-based sample. The study provides an initial indication that ghrelin is also involved in the regulation of alcohol consumption in non-dependent subjects.

Pharmacological manipulation of the ghrelin system and alcohol hangover symptoms in heavy drinking individuals: Is there a link?

Ghrelin, an orexigenic peptide synthesized in the stomach, is a key player in the gut-brain axis. In addition to its role in regulating food intake and energy homeostasis, ghrelin has been shown to modulate alcohol-related behaviors. Alcohol consumption frequently results in hangover, an underexplored phenomenon with considerable medical, psychological, and socioeconomic consequences. While the pathophysiology of hangover is not clear, contributions of mechanisms such as alcohol-induced metabolic/endocrine changes, inflammatory/immune response, oxidative stress, and gut dysbiosis have been reported. Interestingly, these mechanisms considerably overlap with ghrelin's physiological functions. Here, we investigated whether pharmacological manipulation of the ghrelin system may affect alcohol hangover symptoms. Data were obtained from two placebo-controlled laboratory studies. The first study tested the effects of intravenous (IV) ghrelin and consisted of two experiments: a progressive-ratio IV alcohol self-administration (IV-ASA) and a fixed-dose IV alcohol clamp. The second study tested the effects of an oral ghrelin receptor inverse agonist (PF-5190457) and included a fixed-dose oral alcohol administration experiment. Alcohol hangover data were collected the morning after each alcohol administration experiment using the Acute Hangover Scale (AHS). IV ghrelin, compared to placebo, significantly reduced alcohol hangover after IV-ASA (p = 0.04) and alcohol clamp (p = 0.04); PF-5190457 had no significant effect on AHS scores. Females reported significantly higher hangover symptoms than males following the IV-ASA experiment (p = 0.04), but no gender × drug condition (ghrelin vs. placebo) effect was found. AHS total scores were positively correlated with peak subjective responses, including 'stimulation' (p = 0.08), 'sedation' (p = 0.009), 'feel high' (p = 0.05), and 'feel intoxicated' (p = 0.03) during the IV-ASA. IV ghrelin blunted the positive association between alcohol sedation and hangover as shown by trend-level drug × sedation effect (p = 0.08). This is the first study showing that exogenous ghrelin administration, but not ghrelin receptor inverse agonism, affects hangover symptoms. Future research should investigate the potential mechanism(s) underlying this effect.

New steps for treating alcohol use disorder

Alcohol use disorder is a complex syndrome with multiple treatment points including drug-induced pathology, withdrawal management, behavioral/cognitive strategies, and relapse prevention. These different components may be complicated by genotype and phenotype. A huge milestone for the treatment of alcohol use disorder across several countries in the last 10 years was the introduction of practice guidelines integrating clinical expertise and research evidence. These provide a summary of interventions that have been shown to be effective following rigorous and replicated clinical trials. Inspection of these guidelines reveals good consistency, but little evidence of progress in treatment approaches for alcohol use disorder over the past decade. In this mini-review, we discuss emerging treatments for alcohol use disorder that may supplement or improve the evidence-based treatments that are currently recommended. New medications, the emergence of digital technology, and other novel approaches such as transcranial magnetic stimulation are all discussed with reference to treatments already in practice. We also consider how individual differences in genotype and phenotype may affect outcomes. Together with improvements in technology, this knowledge offers a powerful tool for designing personalized approaches to treatment, and hence improving prognosis for rehabilitation programs.

Stress, Motivation, and the Gut-Brain Axis: A Focus on the Ghrelin System and Alcohol Use Disorder

Since its discovery, the gut hormone, ghrelin, has been implicated in diverse functional roles in the central nervous system. Central and peripheral interactions between ghrelin and other hormones, including the stress-response hormone cortisol, govern complex behavioral responses to external cues and internal states. By acting at ventral tegmental area dopaminergic projections and other areas involved in reward processing, ghrelin can induce both general and directed motivation for rewards, including craving for alcohol and other alcohol-seeking behaviors. Stress-induced increases in cortisol seem to increase ghrelin in the periphery, suggesting a pathway by which ghrelin influences how stressful life events trigger motivation for rewards. However, in some states, ghrelin may be protective against the anxiogenic effects of stressors. This critical review brings together a dynamic and growing literature, that is, at times inconsistent, on the relationships between ghrelin, central reward-motivation pathways, and central and peripheral stress responses, with a special focus on its emerging role in the context of alcohol use disorder.

The Role of the Ghrelin System in Drug Addiction

In the past years, a significant volume of research has implicated the appetitive hormone ghrelin in the mechanisms underlying drug use and addiction. From a neuroscientific standpoint, ghrelin modulates both reward and stress pathways, two key drivers of substance use behaviors. Previous investigations support a connection between the ghrelin system and alcohol, stimulants, and tobacco use in both animals and humans, while the research on opioids and cannabis is scarce. In general, upregulation of the ghrelin system seems to enhance craving for drugs as well as substances use. On the other hand, acute and chronic exposure to drugs of abuse influences the ghrelin system at different levels. This chapter summarizes the literature on the relationship between the ghrelin system and substance-related behaviors. We also review recent work investigating the ghrelin system as a potential pharmacological target for treating substance use disorders and discuss the need for additional research.