Within the human society worldwide, substance use disorders, also known as drug addiction, cause serious health, social and economic burden on a long-term basis. Europe ranks first in alcohol and tobacco annual consumptions and attributable mortalities; according to an expert estimation, ~5.5 % of all annual deaths in Europe are alcohol-attributable and ~16% of all deaths in adults over 30 are tobacco-related, with many of these deaths occurring prematurely [1,2]. On the contrary, regarding drug overdose deaths involving substances other than alcohol and tobacco, the USA overtakes the European rates by almost 10-times, with 217 drug overdose deaths per million in the population aged 15–64 compared to 22.6 overdose deaths per million (based on data from 2017, though this trend seems enduring). Recently, the majority of the drug overdoses have been caused by opioids [3,4]. An increasing number of fatal overdoses and related deaths in Europe involving cocaine and amphetamines, synthetic cannabinoids, and fake medicines, such as benzodiazepines have also been reported. Cannabinoids have traditionally been the most widely used illegal drugs in Europe and elsewhere, and the effects of novel synthetic cannabinoids are often more severe than those of cannabis. However, in most overdose and drug-related cases, multiple drug toxicity is implicated [3,4].

Drug addiction dramatically aggravates overall harm and increases the fatal consequences of drug use. The drug intake is determined by drug’s pharmacological rewarding effects and is influenced by many internal (genetics, developmental, co-morbidities etc.) and external (social surroundings, stress, drug availability, social norms and systems etc.) factors. Drug addiction is a chronic relapsing brain disease triggered by repeated drug exposures in individuals who are vulnerable due to genetic predispositions and developmental or adverse social exposures [5,6]. The reinforcing effects of drugs mainly depend on the activation of the mesolimbic dopaminergic system, a crucial part of the brain reward system, which is a substantial component of cardinal human survival strategies. Natural survival-strengthening stimuli, such as food and sex, are accentuated by simultaneous pleasurable feelings linked with dopamine release in the nucleus accumbens (NAC) [7,8,9]. This triggers conditioning with the reward originators and influences motivation, control, and decision-making processing, with participation of the hippocampus, prefrontal cortex and other brain areas, including the amygdala. All currently known drugs of abuse induce the massive release of dopamine in the NAC (shell) through direct or indirect pharmacological effects on the dopaminergic neurons in the ventral tegmental area (VTA) and/or the nucleus accumbens (NAC) [5,6,8,10,11,12,13]. Converging evidence indicates that the excitatory cholinergic terminals in the VTA that arise from the laterodorsal tegmental area (LDTg) synapse on the VTA-dopaminergic neurons, which preferentially innervate the NAC [14]. The cholinergic-dopaminergic circuitry possibly contributes to the net rewarding effects, mandates motivational behaviors, and may participate in drug reinforcement [15,16,17,18,19]. Additional neurotransmitter systems contribute to the drug-induced reinforcing effects, including dopamine-modulating or dopamine-independent mechanisms, e.g., endogenous opioids, endocannabinoids, gamma-aminobutyric acid (GABA), acetylcholine, serotonin, and neuropeptides [20,21,22,23,24,25,26]. The unnaturally excessive drug-induced activation of the pre-set reward/reinforcement system, consequently overburdens the interconnected brain circuits, including the decision- and control-regulating systems. In vulnerable individuals, repeated/chronic drug exposure leads to drug-centric behavior, self-regulation impairment, and addiction.

There is a noticeable overlap of involved neuronal systems [27] as well as a high comorbidity between eating disorders and substance use disorders [28]. The prevalence of substance use was found to be increased with food restriction/dieting severity [29], and food restriction was found to enhance the central rewarding effects of abused drugs [30,31,32], which inspired the investigation into the involvement of appetite regulating substances in drug addiction. Recent studies have already implicated several neuropeptides of the gut-brain axis, such as ghrelin, glucagon-like peptide/GLP-1, amylin, and neuromedin U/NMU, as modulators of reward and proaddictive processes [26]. The only orexigenic gut-brain peptide hormone, ghrelin, commonly called “hunger hormone”, has been extensively studied for more than 20 years since its discovery [33]. It has been documented that ghrelin exerts its potent orexigenic and pro-obesity effects not only through metabolic homeostatic regulatory mechanisms in the hypothalamus, but also by significant increasing food reward and motivation through mesolimbic activation [34,35,36]. Ghrelin predominantly arises from gastric endocrine mucosal cells and is mainly released by an empty stomach [37,38,39]. Small formations of ghrelin have also been found in several peripheral organs (testis, ovary, placenta, kidney, small intestine, pancreas, lymphocytes etc.) as well as in the brain (hypothalamus) [40,41,42]. The notion that ghrelin can be produced in the central nervous system of mammals in physiologically relevant levels still remains controversial [43]. Ghrelin is cleaved from the preproghrelin peptide, which is encoded by the GHRL gene on chromosome 3. Ghrelin, when modified by the ghrelin-O-acyl transferase (GOAT), forms acyl-ghrelin. This n-octanoyl form of ghrelin, acylated at serine 3, activates the growth hormone secretagogue receptor type A1 (GHS-R1A) and releases the growth hormone [33,38]. Thus, acyl-ghrelin is called the active form of ghrelin. The des-acyl ghrelin (without the n-octanoyl moiety) represents the majority form of ghrelin in the blood and behaves as a separate hormone with a number of physiological functions [44,45]. It is mostly inactive at the GHS-R1A but can variously influence the acyl-ghrelin effects [38,45,46,47,48]. Both ghrelin forms are detectable in the blood circulation and can pass through the blood brain barrier to the brain [49,50]. Some studies have indicated that peripheral ghrelin administration might not directly reach the deep brain areas other than the hypothalamus [51,52,53,54]; therefore, it was proposed that alternative indirect mechanisms of the observed peripheral ghrelin effects could also be considered in the mesolimbic areas (e.g., through cholinergic neurons or orexin-containing neurons projecting from hypothalamus to these reward-related areas) [26,55]. Additionally, acyl-ghrelin independent central effects on the ghrelin receptors have been suggested (see below).

Acyl-ghrelin mediates its plentiful peripheral as well as central physiological functions through the GHS-R1A, which is encoded by the GHSR gene on chromosome 3 [56,57]. Through the GHS-R1A, the acylated ghrelin modulates systemic metabolism, food intake, and various other peripheral and central physiological functions, such as inflammation, thermogenesis, cardiac output and contractility, the stimulation of gastric motility, prolactin secretion, depression, sleep-wake rhythm, neuroprotection, memory, and reward behavior [38,58,59,60]. Within the brain, in addition to the hypothalamus, the GHSR1As are expressed in important reward related areas including the striatum, NAC, amygdala, prefrontal cortex, hippocampus, and VTA [35,42,56,61,62,63,64,65,66]. The GHS-R1A is metabotropic and uses several different G-protein secondary-messenger signaling pathways [66,67]. The GHS-R1A also shows high constitutive activity, i.e., intrinsic activity that is independent of ligand binding, that can be reduced by treatment with inverse agonists [58,68,69,70,71]. It is assumed that the ligand-independent activity is also important for several central processes, e.g., reward, memory, and anxiety [54,71]. GHS-R1A oligomerizations and dimerizations have been found with a wide array of other G-protein coupled receptors, including dopamine receptors D1R and D2R, the serotonin 2C receptor (5-HT-2C), and the melanocortin (MC3R, MC4R), somatostatin, oxytocin, and sigma-1 receptors [72,73,74,75]. It was suggested that the GHS-R1A might also form a heterodimer with the cannabinoid CB1 receptor (CB1R) [72]. Furthermore, another variant of the ghrelin receptor, GHS-R1B, forms heterodimers/oligomeric complexes with the GHS-R1A and acts as a dual modulator of its functions [58,76]. In actual fact, (acyl-)ghrelin does not bind to the GHS-R1B; nevertheless, this receptor subtype not only determines the efficacy of ghrelin-induced, GHS-R1A-mediated signaling but also seems to determine the ability of GHS-R1A to form oligomeric complexes with other receptors (e.g., D1R), promoting profound qualitative changes in ghrelin-induced signaling [47,76,77]. However, the functional role of GHS-R1B still remains much less explored [74,78,79]. Recently, the liver-expressed antimicrobial peptide 2 (LEAP2) was described as an endogenous antagonist/inverse agonist of the GHS-R1A which regulates the transmission of intracellular signals from ghrelin [58,80,81].

The strong constitutive activity of the GHS-R1A and its aforementioned ability to heterodimerize with the D1R/D2R in the VTA possibly alter the sensitivity of the mesolimbic dopamine system. Central GHS-R1A receptors are frequently colocalized with dopaminergic and cholinergic receptors [42,57], and it is believed that midbrain functional interactions between these receptors amplify the dopaminergic signaling in the VTA neurons and stimulate the overflow of dopamine in the NAC [76,82,83]. The systemic and central (into the VTA) administration of acyl-ghrelin induced the dopamine release in the NAC (shell) and locomotor hyperactivity in rodents [61,64,82,84,85]. The injection of acyl-ghrelin into the LDTg, where the GHS-R1As are located on the cholinergic neurons, also stimulated the locomotor activity, caused accumbens dopamine efflux, and concomitantly enhanced the levels of the VTA-acetylcholine via the nicotinic cholinergic receptor (nAChR)-dependent mechanism [55,84]. Additionally, the peripheral injection of acyl-ghrelin induced the concomitant release of VTA acetylcholine and accumbens dopamine [55]. These findings suggest that ghrelin stimulates the cholinergic-dopaminergic circuitry, possibly via the nAChR in the VTA, and as previously mentioned, this circuitry is involved in motivational and reward behaviors [18,19]. Thus, ghrelin/GHS-R1A play a crucial role in general reward processing.

Acyl-ghrelin also interacts with the hypothalamic-pituitary-adrenal axis (HPA) and modulates stress-related behaviors [86,87]. On the basis of animal studies, it appears that ghrelin might exert anxiolytic effects in acute stress conditions, while promoting anxiogenic behavior in unstressed and chronically stressed conditions [87,88,89,90,91]. The apparent roles of ghrelin in reward- and stress-related systems suggested the importance of its participation in addiction processes, including drug addiction, which are typically associated with positive (reward) and negative (stress relieve) reinforcements [5,6,82,87]. Indeed, an increasing number of experimental and clinical studies have started to investigate the possible associations between drugs of abuse and ghrelin signaling.

Initial preclinical experiments explored and evidenced the ghrelin’s involvement in the processing of reinforcing effects of alcohol, which can be considered both a palatable food and energy resource and an addictive substance [92]. The relationships between ghrelin/GHS-R1A and alcohol have been extensively studied—see the several recent comprehensive reviews [26,93,94,95]. Based on a significant number of encouraging preclinical results, ghrelin/GHS-R1A antagonism was suggested as a novel treatment approach for alcohol addiction therapy and recently, the first GHS-R1A-related compound was tested in initial human laboratory studies. The GHS-R1A-inverse agonist (PF5190457) was found to be well-tolerated and safe in healthy volunteers [96], and in a single-blind, within-subject, placebo controlled human laboratory study in heavy alcohol drinkers, it was found to significantly reduce the alcohol cue-induced craving in comparison to the placebo, while the safety/tolerability assessment revealed only mild or moderate adverse events, which did not require the discontinuation or dose reduction of the PF5190457 [71,97]. A robust tow-compartment model describing the pharmacokinetic characteristics of PF5190457 has been created [98]. Further research continues.

Gradually, the possible relationships between ghrelin/GHS-R1As and several other drugs of abuse have also been explored, and many interesting results have already been assembled in several reviews [26,99,100,101]. Recent investigation on ghrelin signaling and nonalcohol addictive drugs interactions possibly provided the most novel contribution, and therefore our article focuses on this topic. As previously mentioned, the most advanced and extensive research of alcohol-ghrelin relationships, including an initial clinical research of the potential utilization of ghrelin antagonism for alcohol addiction therapy, has been described in-detail in complex, up-to-date reviews [26,93]. The update of the clinical study using PF5190457 was mentioned above [71,97,98]. Thus, the current review summarizes essential existing preclinical and clinical studies on the interaction between ghrelin/GHS-R1A system and nonalcohol drugs of abuse, specifically nicotine, stimulants, opioids, and cannabinoids.

Drug dependence research uses experimental addiction models. The most important methods that are mentioned in the review are briefly summarized below. The administration of addictive drugs typically induces behavioral stimulation/hyperlocomotion in rodents, involving stereotypical behaviors (licking, confined gnowing, sterotyped sniffing, etc.), which is considered to be a sign of nigrostriatal dopaminergic pathway activation [6,19]. Furthermore, behavioral sensitization (associated with accumbens dopaminergic sensitization), which is caused by repeated addictive drug administration, is considered within certain limits to be a manifestation of the incentive motivation of underlying drug-seeking behavior [18,102,103]. Incidentally, the subchronic systemic ghrelin treatment itself has also induced locomotor sensitization [104], which corresponds with ghrelin’s reinforcement ability. The drug-conditioned place preference (CPP) method is mainly focused on association/conditioning of environmental cues with a substance effect, which plays an important role in the acquisition and maintenance of addiction [105,106,107]. A crucial experimental addiction model, intravenous self-administration (IVSA), is used to assess the rewarding and reinforcing effects of drugs and to evaluate the principle treatment goal of abolishing/reducing drug consumption [107]. A specific IVSA arrangement with a period of abstinence allows the assessment of the tendency to relapse, i.e., drug-seeking behavior [108,109]. Ghrelin signaling and blood levels are affected by many factors [110], including satiety/hunger status. Circulating ghrelin is enhanced following hunger/food deprivation [38,39,111,112]; thus, the food conditions might have impacted some of the results mentioned in this review. In the majority of the experimental studies reviewed in this paper that tested the effects of addictive drugs, the animals had free access to standard food, which usually was removed during testing (unless otherwise specified). In clinical studies, the food conditions were more heterogenous.

In this review, we summarize our current knowledge from preclinical and clinical studies within the ghrelin/nonalcohol drugs of abuse research, highlight the imperfections, and suggest possible directions for future research.