Ghrelin receptor antagonism of fentanyl-induced conditioned place preference, intravenous self-administration, and dopamine release in the nucleus accumbens in rats

The synthetic opioid isotonitazene induces locomotor activity and reward effects through modulation of the central dopaminergic system in mice

Synthetic opioids, which differ from fentanyl, have recently emerged as new psychoactive substances and pose public health concerns. The pharmacological effects and drug dependency of these benzimidazole-based opioids, also known as nitazenes, remain unclear. In this study, we examined the selectivity of opioid receptors, effects on motor activity, and expression of reward effects for isotonitazene, which has been detected in many poisonings and fatalities since 2019. Isotonitazene was most selective for the μ-receptor and exhibited more potent agonist effects, with an EC50 of 0.02 nM, than morphine (EC50 = 34 nM) and fentanyl (EC50 = 4.0 nM). In ICR mice, isotonitazene (up to 0.05 mg/kg) increased the locomotor activity in a dose-dependent manner. This effect was significantly suppressed by pretreatment with the opioid receptor antagonists naloxone (3 mg/kg) and β-FNA (1 mg/kg), the dopamine D1 receptor antagonist SCH23390 (0.5 mg/kg), and dopamine D2 receptor antagonist raclopride (6 mg/kg). The reward effects of isotonitazene, evaluated using conditioned place preference (CPP) in mice, showed that conditioning with isotonitazene produced significant dose-dependent CPP scores. Microdialysis analysis also confirmed that the isotonitazene dose that induced CPP (0.05 mg/kg) significantly increased dopamine levels in the nucleus accumbens of mice. These results suggest that isotonitazene, similar to fentanyl and morphine, is a compound with a high risk of forming drug dependence and reward effects via the dopaminergic nervous system. This study provides foundational data for biological evaluation of other nitazene compounds.

Assessment of abuse potential of furanylfentanyl and tetrahydrofuranylfentanyl

RATIONAL

Furanylfentanyl and tetrahydrofuranylfentanyl (THF-F) have been emerging in numerous intoxication and overdose cases in recent years. However, there remains a data deficiency regarding the abuse potential of these novel fentanyl analogs.

OBJECTIVES

This study was designed to systematically assess the abuse potential of furanylfentanyl and THF-F.

METHODS

In this study, we evaluated the abuse potential of furanylfentanyl and THF-F via the conditioned place preference (CPP), drug self-administration, drug discrimination, and naloxone-precipitated withdrawal experiments with fentanyl as a reference.

RESULTS

Results from CPP experiments indicated that furanylfentanyl and THF-F could induce CPP at minimum doses of 0.1 mg/kg and 3 mg/kg, respectively. These doses were 1 time and 30 times that of fentanyl (0.1 mg/kg). Furanylfentanyl elicited stable self-administration responses at 2.5 µg/kg/infusion, whereas THF-F did so at 50 µg/kg/infusion. In the drug-substitution test, furanylfentanyl and THF-F induced the maximum number of infusions at 1.10 µg/kg and 12.5 µg/kg, respectively, which were 1 time and 10 times that of fentanyl (1.21 µg/kg). In drug discrimination tests, all three substances were fully substituted for the discriminative-stimulus effects of heroin dose-dependently. The substitution potency of furanylfentanyl (ED50 = 2.68 µg/kg) was similar to that of fentanyl (ED50 = 2.66 µg/kg), while THF-F (ED50 = 36.32 µg/kg) was 14-fold less potent than fentanyl. Repeated administration of furanylfentanyl and THF-F produced naloxone-precipitated withdrawal symptoms. Thus, furanylfentanyl exhibited comparable potency to fentanyl in terms of rewarding, reinforcing, and subjective effects, while THF-F had reduced potency in these effects. Both of them had physical dependence.

CONCLUSIONS

Taken together, our study presented new evidence indicating that furanylfentanyl and THF-F exhibit significant abuse potential in rodent models, which provides experimental data for the control. Furthermore, our study offered valuable information for future studies into the addictive properties of structurally modified fentanyl analogs.

Effects of Fentanyl-Laced Cocaine on Circulating Ghrelin, Insulin, and Glucose Levels in Rats

Opioid mixed with cocaine has been increasingly implicated in overdose deaths, including both intentional co-use of opioid and cocaine and fentanyl-adulterated drug supply. As ghrelin plays an important role in drug reward and can also influence insulin, this study aimed to assess responses of the circulating ghrelin, insulin, and glucose levels to the combined use of fentanyl and cocaine (a polydrug) in rats by performing combined animal behavioral experiments and biochemical analysis. The experimental data consistently revealed that the fentanyl and cocaine co-use can significantly elevate both the acyl-ghrelin and desacyl-ghrelin levels and significantly decrease the insulin level without significant effects on the glucose level. These findings suggest that, like cocaine itself, the fentanyl-cocaine polydrug can self-promote its rewarding effects via elevating the ghrelin level, and that the ghrelin system might serve as a potential pharmacological target for treatment of substance use disorders caused by polysubstance use involving fentanyl and cocaine. Additionally, based on the insulin data obtained in this study, the insulin level was always downregulated significantly and considerably, implying that the fentanyl and cocaine polydrug might have a stronger cardiovascular toxicity to the patients with insulin resistance and diabetes. Further studies are required to examine this possibility.

Interactions of pain and opioids on conditioned place preference in rodents

RATIONALE

Opioid analgesics are the most effective medications used for the treatment of pain, however there are significant risks associated with repeated opioid use including opioid misuse and opioid use disorder development. Chronic pain affects millions of adults in the United States, and opioid misuse is often comorbid with pain conditions in individuals who are repeatedly treated with opioids. In addition to providing pain relief, opioids produce rewarding effects, but in chronic pain states, reward processing can become dysregulated. The conditioned place preference task is commonly used to measure the rewarding properties of opioids in rodents. During this task, opioid administration is paired with a distinct environment through repeated conditioning and the change in an animal's preference for the paired environment indicates whether the opioid is rewarding or not.

OBJECTIVES

Rodent pain models can be combined with conditioned place preference to examine the effects of pain on opioid reward. The existing preclinical literature on pain effects on conditioned place preference is conflicting, where pain conditions have been reported to enhance, suppress, or have no effect on opioid reward. This review will discuss several factors that may contribute to these discordant findings including conditioning session duration and number, rodent strain differences in opioid sensitivity, analgesic properties of opioids at tested doses, locomotor effects at tested doses, and diurnal variation in pain sensitivity. Future studies should consider how these factors contribute to opioid conditioned place preference in both pain and pain-free animals to have a better understanding of the interactions between pain and opioid reward.

Isobutyryl-carfentanyl has strong acute toxicity and analgesic effects with high addiction potential

RATIONALE

Isobutyryl-carfentanyl is the most recently discovered fentanyl analogue with a chemical structure that is similar to that of carfentanyl. Its analogue, carfentanyl, is regarded as one of the most lethal drugs in the world, with a potency of 10,000 times that of morphine. Therefore, isobutyryl-carfentanyl may possess a comparably high potency and its harmful effects cannot be ignored.

OBJECTIVES

This study was designed to assess the analgesic effect of isobutyryl-carfentanyl and the potential risks associated with its misuse.

METHODS

In this study, we assessed the acute toxicity of isobutyryl-carfentanyl by up-and-down-procedure, the analgesic efficacy by hot-plate test, the abuse potential by conditioned place preference (CPP), drug self-administration, and drug discrimination tests, and compared it with fentanyl and carfentanyl.

RESULTS

The estimated median lethal dose (LD50) of isobutyryl-carfentanyl administered were 175 mg/kg (intragastric administration, IG), 15.84 mg/kg (intraperitoneal injection, IP), 15.84 mg/kg (subcutaneous injection, SC), and 1.6 mg/kg (intravenous injection, IV), respectively. The 50% maximal analgesic effect (ED50) of isobutyryl-carfentanyl was determined to be 0.00319 mg/kg, with an analgesic potency 14 times that of fentanyl and 0.82 times that of carfentanyl. Isobutyryl-carfentanyl exhibited a significant positional preference at a minimum dose of 0.1 mg/kg, while fentanyl exhibited a significant positional preference at a minimum dose of 0.3 mg/kg. In the heroin (0.05 mg/kg/infusion) self-administration substitution experiment, isobutyryl-carfentanyl showed significant self-administration behaviour at doses of 0.0005-0.001 mg/kg/infusion, with the maximum number of infusions observed at a dose of 0.001 mg/kg. In the heroin (1 mg/kg) drug discrimination experiment, fentanyl (0.005-0.02 mg/kg), carfentanyl (0.0005-0.002 mg/kg), and isobutyryl-carfentanyl (0.001-0.005 mg/kg) were tested in the dose-effect curves. The results showed that all three drugs exhibit dose-dependent increase in the number of drug-associated nose pokes responses and reduction in the rate of nose pokes. The subjective effect potency of isobutyryl-carfentanyl was found to be 4.4 times that of fentanyl and 0.5 times that of carfentanyl.

CONCLUSIONS

In summary, isobutyryl-carfentanyl has high acute toxicity and analgesic effect, with strong psychological dependence approximately 5 times that of fentanyl and 0.5 times that of carfentanyl, and has extremely high abuse potency.

Dopamine D2 receptors in the accumbal core region mediates the effects of fentanyl on sleep-wakefulness

Fentanyl, a potent analgesic and addictive substance, significantly impacts sleep-wakefulness (S-W). Acutely, it promotes wake, whereas chronic abuse leads to severe sleep disruptions, including insomnia, which contributes to opioid use disorders (OUD), a chronic brain disease characterized by compulsive opioid use and harmful consequences. Although the critical association between sleep disruptions and fentanyl addiction is acknowledged, the precise mechanisms through which fentanyl influences sleep remain elusive. Recent studies highlight the role of the dopaminergic system of the nucleus accumbens (NAc) in S-W regulation, but its specific involvement in mediating fentanyl's effects on S-W remains unexplored. We hypothesized that dopamine D2 receptors mediate fentanyl-induced effects on S-W. To test this hypothesis, male C57BL/6J mice, instrumented with sleep recording electrodes and bilateral guide cannulas above the accumbal core region (NAcC), were utilized in this study. At dark onset, animals were bilaterally administered sulpiride (D2 receptors antagonist; 250 ng/side) in the NAcC followed by an intraperitoneal injection of fentanyl (1.2 mg/Kg). S-W was examined for the next 12 h. We found that systemic administration of fentanyl significantly increased wakefulness during the first 6 h of the dark which was followed by a significant increase in NREM and REM sleep during the second 6 h of the dark period. D2-receptor blockade significantly reduced this effect as evidenced by a significant reduction in fentanyl-induced wakefulness during first 6 h of dark period and sleep rebound during the second 6 h. Our findings suggest that D2 receptors in the NAcC plays a vital role in mediating the fentanyl-induced changes in S-W.

Contribution of Visceral Systems to the Development of Substance Use Disorders: Translational Aspects of Interaction between Central and Peripheral Mechanisms

Substance use disorders are associated with structural and functional changes in the neuroendocrine, neuromediator, and neuromodulator systems in brain areas involved in the reward and stress response circuits. Chronic intoxication provokes emergence of somatic diseases and aggravates existing pathologies. Substance use disorders and somatic diseases often exacerbate the clinical courses of each other. Elucidation of biochemical pathways common for comorbidities may serve as a basis for the development of new effective pharmacotherapy agents, as well as drug repurposing. Here, we discussed molecular mechanisms underlying integration of visceral systems into the central mechanisms of drug dependence.

In vitro pharmacological characterization of growth hormone secretagogue receptor ligands using the dynamic mass redistribution and calcium mobilization assays

Ghrelin modulates several biological functions via selective activation of the growth hormone secretagogue receptor (GHSR). GHSR agonists may be useful for the treatment of anorexia and cachexia, while antagonists and inverse agonists may represent new drugs for the treatment of metabolic and substance use disorders. Thus, the identification and pharmacodynamic characterization of new GHSR ligands is of high interest. In the present work the label-free dynamic mass redistribution (DMR) assay has been used to evaluate the pharmacological activity of a panel of GHSR ligands. This includes the endogenous peptides ghrelin, desacyl-ghrelin and LEAP2(1-14). Among synthetic compounds, the agonists anamorelin and HM01, the antagonists HM04 and YIL-781, and the inverse agonist PF-05190457 have been tested, together with HM03, R011, and H1498 from patent literature. The DMR results have been compared to those obtained in parallel experiments with the calcium mobilization assay. Ghrelin, anamorelin, HM01, and HM03 behaved as potent full GHSR agonists. YIL-781 behaved as a partial GHSR agonist and R011 as antagonist in both the assays. LEAP2(1-14) resulted a GHSR inverse agonist in DMR but not in calcium mobilization assay. PF-05190457, HM04, and H1498 behaved as GHSR inverse agonists in DMR experiments, while they acted as antagonists in calcium mobilization studies. In conclusion, this study provided a systematic pharmacodynamic characterization of several GHSR ligands in two different pharmacological assays. It demonstrated that the DMR assay can be successfully used particularly to discriminate between antagonists and inverse agonists. This study may be useful for the selection of the most appropriate compounds to be used in future studies.

Diverse Underlying Mechanisms and Sex Differences Found in Translational Models of Cannabinoids Use: Towards Validation in Human Studies

This Special Issue represents a continuation of our previous Special Issue entitled "Endocannabinoids, Cannabinoids and Psychiatry: Biological Mechanisms" [...].

Ghrelin receptor antagonist JMV2959 blunts cocaine and oxycodone drug-seeking, but not self-administration, in male rats

The drug overdose crisis has spawned serious health consequences, including the increased incidence of substance use disorders (SUDs), conditions manifested by escalating medical and psychological impairments. While medication management is a key adjunct in SUD treatment, this crisis has crystallized the need to develop additional therapeutics to facilitate extended recovery from SUDs. The "hunger hormone" ghrelin acts by binding to the growth hormone secretagogue receptor 1α (GHS1αR) to control homeostatic and hedonic aspects of food intake and has been implicated in the mechanisms underlying SUDs. Preclinical studies indicate that GHS1αR antagonists and inverse agonists suppress reward-related signaling associated with cocaine and opioids. In the present study, we found that the GHS1αR antagonist JMV2959 was efficacious to suppress both cue-reinforced cocaine and oxycodone drug-seeking, but not cocaine or oxycodone self-administration in male Sprague-Dawley rats. These data suggest a role of the ghrelin-GHS1αR axis in mediating overlapping reward-related aspects of cocaine and oxycodone and premises the possibility that a GHS1αR antagonist may be a valuable therapeutic strategy for relapse vulnerability in SUDs.

Claustral neurons projecting to frontal cortex restrict opioid consumption

The synthetic opioid fentanyl is a major contributor to the current opioid addiction crisis. We report that claustral neurons projecting to the frontal cortex limit oral fentanyl self-administration in mice. We found that fentanyl transcriptionally activates frontal-projecting claustrum neurons. These neurons also exhibit a unique suppression of Ca²⁺ activity upon initiation of bouts of fentanyl consumption. Optogenetic stimulation of frontal-projecting claustral neurons, intervening in this suppression, decreased bouts of fentanyl consumption. In contrast, constitutive inhibition of frontal-projecting claustral neurons in the context of a novel, group-housed self-administration procedure increased fentanyl bout consumption. This same manipulation also sensitized conditioned-place preference for fentanyl and enhanced the representation of fentanyl experience in the frontal cortex. Together, our results indicate that claustrum neurons exert inhibitory control over frontal cortical neurons to restrict oral fentanyl intake. Upregulation of activity in the claustro-frontal projection may be a promising strategy for reducing human opioid addiction.

Chronic inflammatory pain promotes place preference for fentanyl in male rats but does not change fentanyl self-administration in male and female rats

The current opioid epidemic is a national health crisis marked by skyrocketing reports of opioid misuse and overdose deaths. Despite the risks involved, prescription opioid analgesics are the most powerful and effective medications for treating pain. There is a clear need to investigate the risk of opioid misuse liability in male and female adults experiencing chronic pain. In the present study, we tested the hypothesis that chronic inflammatory pain would increase fentanyl intake, motivation to acquire fentanyl, and drug seeking in the absence of fentanyl in rats. Fentanyl intake, motivation for fentanyl, and drug seeking were tested under limited and extended access conditions using intravenous fentanyl self-administration. Fos activity in ventral tegmental area (VTA) dopamine neurons following intravenous fentanyl challenge (35 μg/kg) was examined using immunohistochemistry. Finally, we tested whether low-dose fentanyl supports development of conditioned place preference under an inflammatory pain state in rats. Contrary to our hypothesis, fentanyl self-administration and VTA Fos activity were unaffected by inflammatory pain status. During acquisition, males exhibited increased fentanyl intake compared to females. Animals given extended access to fentanyl escalated fentanyl intake over time, while animals given limited access did not. Males given extended access to fentanyl demonstrated a greater increase in fentanyl intake over time compared to females. During the dose-response test, females given limited access to fentanyl demonstrated increased motivation to acquire fentanyl compared to males. Both sexes displayed significant increases in responding for fentanyl as unit fentanyl doses were lowered. Following fentanyl challenge, females exhibited higher numbers of Fos-positive non-dopaminergic VTA neurons compared to males. Using conditioned place preference, we found that chronic inflammatory pain promotes fentanyl preference in males, but not females. These findings suggest that established fentanyl self-administration is resistant to change by inflammatory pain manipulation in both sexes, but chronic inflammatory pain increases the rewarding properties of low-dose fentanyl in males.

Ghrelin/GHS-R1A antagonism in memory test and its effects on central molecular signaling involved in addiction in rats

Central ghrelin signaling seems to play important role in addiction as well as memory processing. Antagonism of the growth hormone secretagogue receptor (GHS-R1A) has been recently proposed as a promising tool for the unsatisfactory drug addiction therapy. However, molecular aspects of GHS-R1A involvement in specific brain regions remain unclear. The present study demonstrated for the first time that acute as well as subchronic (4 days) administration of the experimental GHS-R1A antagonist JMV2959 in usual intraperitoneal doses including 3 mg/kg, had no influence on memory functions tested in the Morris Water Maze in rats as well as no significant effects on the molecular markers linked with memory processing in selected brain areas in rats, specifically on the β-actin, c-Fos, two forms of the calcium/calmodulin-dependent protein kinase II (CaMKII, p-CaMKII) and the cAMP-response element binding protein (CREB, p-CREB), within the medial prefrontal cortex (mPFC), nucleus accumbens (NAc), dorsal striatum, and hippocampus (HIPP). Furthermore, following the methamphetamine intravenous self-administration in rats, the 3 mg/kg JMV2959 pretreatment significantly reduced or prevented the methamphetamine-induced significant decrease of hippocampal β-actin and c-Fos as well as it prevented the significant decrease of CREB in the NAC and mPFC. These results imply, that the GHS-R1A antagonist/JMV2959 might reduce/prevent some of the memory-linked molecular changes elicited by methamphetamine addiction within brain structures associated with memory (HIPP), reward (NAc), and motivation (mPFC), which may contribute to the previously observed significant JMV2959-induced reduction of the methamphetamine self-administration and drug-seeking behavior in the same animals. Further research is necessary to corroborate these results.

Striatal μ-opioid receptor activation triggers direct-pathway GABAergic plasticity and induces negative affect

Withdrawal from chronic opioid use often causes hypodopaminergic states and negative affect, which may drive relapse. Direct-pathway medium spiny neurons (dMSNs) in the striatal patch compartment contain μ-opioid receptors (MORs). It remains unclear how chronic opioid exposure and withdrawal impact these MOR-expressing dMSNs and their outputs. Here, we report that MOR activation acutely suppressed GABAergic striatopallidal transmission in habenula-projecting globus pallidus neurons. Notably, withdrawal from repeated morphine or fentanyl administration potentiated this GABAergic transmission. Furthermore, intravenous fentanyl self-administration enhanced GABAergic striatonigral transmission and reduced midbrain dopaminergic activity. Fentanyl-activated striatal neurons mediated contextual memory retrieval required for conditioned place preference tests. Importantly, chemogenetic inhibition of striatal MOR+ neurons rescued fentanyl withdrawal-induced physical symptoms and anxiety-like behaviors. These data suggest that chronic opioid use triggers GABAergic striatopallidal and striatonigral plasticity to induce a hypodopaminergic state, which may promote negative emotions and relapse.

Assessment of abuse potential of carfentanil

Carfentanil, as a fentanyl analogue, is a potent synthetic opioid. It has been controlled in many countries, and its emergence has been highlighted by many recent reports. However, although discriminative stimulus effects of carfentanil in rats had been reported, its abuse potential has not been fully evaluated. In this study, we evaluated the abuse potential of carfentanil via the tests of conditioned place preference (CPP), drug self-administration and naloxone-precipitated opioid withdrawal assay, compared with fentanyl and heroin. Carfentanil exhibited significant place preference at a minimum dose of 1 μg/kg in mice, whereas fentanyl and heroin induced significant place preference at the minimum doses of 100 μg/kg and 1000 μg/kg, respectively. In the drug-substitution test in heroin self-administered rats (50 μg/kg/infusion), carfentanil and fentanyl acquired significant self-administrations above saline levels from 0.05-0.1 and 0.1-10.0 μg/kg/infusion, respectively. Carfentanil induced the maximum number of infusions at 0.1 μg/kg, whereas fentanyl and heroin at 1 and 25 μg/kg, respectively. In short, carfentanil showed the highest potency to induce CPP and self-administration. Furthermore, repeated treatment with escalating doses of carfentanil, fentanyl or heroin induced typical withdrawal symptoms in mice, including a greater number of jumping and weight loss than saline group. This indicated that carfentanil could produce physical dependence similar to fentanyl and heroin. Taken together, the present study demonstrated the higher abuse potential of carfentanil compared with fentanyl and heroin. The rank order of abuse potential for these compounds is carfentanil > fentanyl > heroin.

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.

Acute treatment with the glucagon-like peptide-1 receptor agonist, liraglutide, reduces cue- and drug-induced fentanyl seeking in rats

Opioid Use Disorder (OUD) is a chronic relapsing disorder that has severe negative impacts on the individual, the family, and the community at large. In 2021, opioids contributed to nearly 70% of all drug overdose deaths in the United States. This rise in opioid related deaths coincides with a significant rise in the use of fentanyl, a synthetic opioid that is 150 times more potent than morphine. Furthermore, this overdose trend has spared no demographic and costs the nation an estimated $51.2 billion annually. Thus, it is imperative to better understand the underlying mechanisms of OUD in an effort to identify new treatment targets. Using animal models, studies have shown that rats readily self-administer heroin and increase seeking following exposure to cues for drug, the drug itself, or stress. We have shown that treatment with the glucagon-like peptide-1 receptor (GLP-1R) agonist, liraglutide, can reduce heroin taking and seeking behavior in rats. Therefore, using our rodent model, we established a fentanyl self-administration paradigm to test whether acute treatment with the GLP-1R agonist also can reduce fentanyl seeking in fentanyl experienced rats. The results showed that rats readily self-administer fentanyl (2.5 ug/kg) intravenously, with marked individual differences in drug taking behavior. As with other drugs of abuse tested, rats exhibited high seeking behavior when challenged with a drug-related cue or, after a period of extinction, the drug itself. Here, acute treatment with the GLP-1R agonist, liraglutide (0.3mg/kg s.c.), was found to attenuate both cue-induced fentanyl seeking and drug-induced reinstatement of fentanyl seeking with the same efficacy as the currently approved partial opioid agonist, buprenorphine. Taken together, these data suggest that a known satiety signal, GLP-1, may serve as an effective non-opioid alternative for the treatment of OUD.

Reinforcing effects of fentanyl and sufentanil aerosol puffs in rats

RATIONALE

Rapidly evolving e-cigarette technology developed for self-administering nicotine aerosol has the potential to be utilized to self-administer other aerosolized drugs of abuse. Rodent models which mirror characteristics of human e-cigarette use are necessary to explore the degree to which this may be a public health concern.

OBJECTIVES

Our goal was to develop a highly translational model of discrete nose-only aerosol puff drug delivery to explore the reinforcing effects of fentanyl and sufentanil aerosols in rats.

METHODS

Male and female Sprague-Dawley rats were trained to perform a multiple schedule FR1 lever-press, 4-s (second) nose hold operant during which the subject's orofacial areas were exposed to drug-free glycerol/propylene glycol aerosol produced by a commercial e-cigarette at a power setting of 18 watts. Each completed 4-s drug-free vehicle aerosol exposure resulted in a 3-s presentation of a 0.1-ml dipper of sweetened milk solution. After training, rats were then allowed to self-administer 4-s nose-only puffs of fentanyl (100-6000 µg/ml) or sufentanil (30-500 µg/ml) aerosol in the absence of paired milk dipper reinforcers.

RESULTS

All 31 rats learned the lever-press/nose-poke multiple schedule for milk dippers alone and 25 accepted exposure to 4 s of 18 watts of drug-free vehicle aerosol when paired with milk dipper presentations. In the absence of paired milk dipper presentations, fentanyl aerosol puffs at concentrations of 1000 and 3000 µg/ml as well as 100 µg/ml puffs of sufentanil served as reinforcers compared to both air puffs and drug-free vehicle aerosol puffs. There were no significant differences between males and females in number of fentanyl or sufentanil puffs self-administered.

CONCLUSIONS

Discrete nose-only puffs of two potent opioids under exposure conditions comparable to puff durations in human e-cigarette users serve as reinforcers in rats. This outcome suggests that under appropriate conditions e-cigarettes might be a potential alternative delivery mechanism for illicit opioids.

Unique Pharmacology, Brain Dysfunction, and Therapeutic Advancements for Fentanyl Misuse and Abuse

Fentanyl is a fully synthetic opioid with analgesic and anesthetic properties. It has become a primary driver of the deadliest opioid crisis in the United States and elsewhere, consequently imposing devastating social, economic, and health burdens worldwide. However, the neural mechanisms that underlie the behavioral effects of fentanyl and its analogs are largely unknown, and approaches to prevent fentanyl abuse and fentanyl-related overdose deaths are scarce. This review presents the abuse potential and unique pharmacology of fentanyl and elucidates its potential mechanisms of action, including neural circuit dysfunction and neuroinflammation. We discuss recent progress in the development of pharmacological interventions, anti-fentanyl vaccines, anti-fentanyl/heroin conjugate vaccines, and monoclonal antibodies to attenuate fentanyl-seeking and prevent fentanyl-induced respiratory depression. However, translational studies and clinical trials are still lacking. Considering the present opioid crisis, the development of effective pharmacological and immunological strategies to prevent fentanyl abuse and overdose are urgently needed.

Involvement of the ghrelin system in the maintenance of oxycodone self-administration: converging evidence from endocrine, pharmacologic and transgenic approaches

Ghrelin, an orexigenic hormone, has emerged as a critical biological substrate implicated in drug reward. However, the response of the ghrelin system to opioid-motivated behaviors and the role of ghrelin in oxycodone self-administration remain to be studied. Here, we investigated the reciprocal interactions between the endogenous ghrelin system and oxycodone self-administration behaviors in rats and the role of the ghrelin system in brain stimulation reward (BSR) driven by optogenetic stimulation of midbrain reward circuits in mice. Oxycodone self-administration significantly elevated plasma ghrelin, des-acyl ghrelin and growth hormone and showed no effect on plasma LEAP2, a newly identified endogenous ghrelin receptor (GHS-R1a) antagonist. Oxycodone self-administration produced significant decreases in plasma gastric inhibitory polypeptide and insulin. Acquisition of oxycodone self-administration significantly upregulated GHS-R1a mRNA levels in dopamine neurons in the ventral tegmental area (VTA), a brain region critical in drug reward. Pretreatment with JMV2959, a selective GHS-R1a antagonist, dose-dependently reduced oxycodone self-administration and decreased the breakpoint for oxycodone under a progressive ratio reinforcement in Long-Evans rats. The inhibitory effects of JMV2959 on oxycodone self-administration is selectively mediated by GHS-R1a as JMV2959 showed a similar effect in Wistar wildtype but not in GHS-R knockout rats. JMV2959 pretreatment significantly inhibited BSR driven by selective stimulation of VTA dopamine neurons, but not by stimulation of striatal GABA neurons projecting to the VTA in mice. These findings suggest that elevation of ghrelin signaling by oxycodone or oxycodone-associated stimuli is a causal process by which oxycodone motivates oxycodone drug-taking and targeting the ghrelin system may be a viable treatment approach for opioid use disorders.

Advances in the Development of Nonpeptide Small Molecules Targeting Ghrelin Receptor

Ghrelin is an octanoylated peptide acting by the activation of the growth hormone secretagogue receptor, namely, GHS-R1a. The involvement of ghrelin in several physiological processes, including stimulation of food intake, gastric emptying, body energy balance, glucose homeostasis, reduction of insulin secretion, and lipogenesis validates the considerable interest in GHS-R1a as a promising target for the treatment of numerous disorders. Over the years, several GHS-R1a ligands have been identified and some of them have been extensively studied in clinical trials. The recently resolved structures of GHS-R1a bound to ghrelin or potent ligands have provided useful information for the design of new GHS-R1a drugs. This perspective is focused on the development of recent nonpeptide small molecules acting as GHS-R1a agonists, antagonists, and inverse agonists, bearing classical or new molecular scaffolds, as well as on radiolabeled GHS-R1a ligands developed for imaging. Moreover, the pharmacological effects of the most studied ligands have been discussed.

Effects of Motivational Downshifts on Specific Pavlovian-Instrumental Transfer in Rats

BACKGROUND

Pavlovian stimuli predictive of appetitive outcomes can exert a powerful influence on the selection and initiation of action, a phenomenon termed outcome-selective Pavlovian-instrumental transfer (sPIT). Rodent studies suggest that sPIT is insensitive to motivational downshift induced by outcome devaluation, an effect that is, however, relatively underexplored.

METHODS

Here we examined in detail the effects of distinct shifts in motivation from hunger to a state of relative satiety on sPIT in rats.

RESULTS

A motivational downshift by outcome-specific devaluation immediately prior to testing markedly reduced overall lever responding and magazine entries but left intact the sPIT effect. A motivational downshift prior testing by (1) giving ad libitum rather than restricted access to maintenance diet in the home cage for 24 hours or by (2) a systemic blockade of hormone secretagogue receptor subtype 1A receptors to inhibit orexigenic actions of ghrelin both reduced overall lever responding and magazine entries. Moreover, these latter motivational downshifts reduced the sPIT effect; however, the sizes of the sPIT effects were still large.

CONCLUSIONS

Collectively, our rodent findings indicate that major effects of various motivational downshifts are overall inhibition of lever pressing and magazine approach, possibly reflecting reduced general motivation. The observed effects of motivational downshifts on sPIT have implications with regard to the role of general motivating effects in sPIT and to the contribution of Pavlovian-instrumental interactions to excessive food seeking as well as obesity in humans.

Substance abuse and neurotransmission

The number of people who suffer from a substance abuse disorder has continued to rise over the last decade; particularly, the number of drug-related overdose deaths has sharply increased during the COVID-19 pandemic. Converging lines of clinical observations, supported by imaging and neuropsychological performance testing, have demonstrated that substance abuse-induced dysregulation of neurotransmissions in the brain is critical for development and expression of the addictive properties of abused substances. Recent scientific advances have allowed for better understanding of the neurobiological processes that mediates drugs of abuse and addiction. This chapter presents the past classic concepts and the recent advances in our knowledge about how cocaine, amphetamines, opioids, alcohol, and nicotine alter multiple neurotransmitter systems, which contribute to the behaviors associated with each drug. Additionally, we discuss the interactive effects of HIV-1 or COVID-19 and substance abuse on neurotransmission and neurobiological pathways. Finally, we introduce therapeutic strategies for development of pharmacotherapies for substance abuse disorders.

The Role of Ghrelin/GHS-R1A Signaling in Nonalcohol Drug Addictions

Drug addiction causes constant serious health, social, and economic burden within the human society. The current drug dependence pharmacotherapies, particularly relapse prevention, remain limited, unsatisfactory, unreliable for opioids and tobacco, and even symptomatic for stimulants and cannabinoids, thus, new more effective treatment strategies are researched. The antagonism of the growth hormone secretagogue receptor type A (GHS-R1A) has been recently proposed as a novel alcohol addiction treatment strategy, and it has been intensively studied in experimental models of other addictive drugs, such as nicotine, stimulants, opioids and cannabinoids. The role of ghrelin signaling in these drugs effects has also been investigated. The present review aims to provide a comprehensive overview of preclinical and clinical studies focused on ghrelin's/GHS-R1A possible involvement in these nonalcohol addictive drugs reinforcing effects and addiction. Although the investigation is still in its early stage, majority of the existing reviewed experimental results from rodents with the addition of few human studies, that searched correlations between the genetic variations of the ghrelin signaling or the ghrelin blood content with the addictive drugs effects, have indicated the importance of the ghrelin's/GHS-R1As involvement in the nonalcohol abused drugs pro-addictive effects. Further research is necessary to elucidate the exact involved mechanisms and to verify the future potential utilization and safety of the GHS-R1A antagonism use for these drug addiction therapies, particularly for reducing the risk of relapse.

An Overview of Appetite-Regulatory Peptides in Addiction Processes; From Bench to Bed Side

There is a substantial need for new pharmacological treatments of addiction, and appetite-regulatory peptides are implied as possible candidates. Appetite regulation is complex and involves anorexigenic hormones such as glucagon-like peptide-1 (GLP-1) and amylin, and orexigenic peptides like ghrelin and all are well-known for their effects on feeding behaviors. This overview will summarize more recent physiological aspects of these peptides, demonstrating that they modulate various aspects of addiction processes. Findings from preclinical, genetic, and experimental clinical studies exploring the association between appetite-regulatory peptides and the acute or chronic effects of addictive drugs will be introduced. Short or long-acting GLP-1 receptor agonists independently attenuate the acute rewarding properties of addictive drugs or reduce the chronic aspects of drugs. Genetic variation of the GLP-1 system is associated with alcohol use disorder. Also, the amylin pathway modulates the acute and chronic behavioral responses to addictive drugs. Ghrelin has been shown to activate reward-related behaviors. Moreover, ghrelin enhances, whereas pharmacological or genetic suppression of the ghrelin receptor attenuates the responses to various addictive drugs. Genetic studies and experimental clinical studies further support the associations between ghrelin and addiction processes. Further studies should explore the mechanisms modulating the ability of appetite-regulatory peptides to reduce addiction, and the effects of combination therapies or different diets on substance use are warranted. In summary, these studies provide evidence that appetite-regulatory peptides modulate reward and addiction processes, and deserve to be investigated as potential treatment target for addiction.

Growth Hormone Secretagogue Receptor 1A Antagonist JMV2959 Effectively Prevents Morphine Memory Reconsolidation and Relapse

Relapse to drug seeking after prolonged abstinence is a major problem in the clinical treatment of drug addiction. The use of pharmacological interventions to disrupt established drug reward memories is a promising strategy for the treatment of drug addiction. A growth hormone secretagogue receptor 1 A antagonist, JMV2959, has been shown to reduce morphine-induced conditioned place preference (CPP) in rats within hours of intervention; thus, JMV2959 is a potential candidate for drug addiction treatment. However, the effect of JMV2959 on reconsolidation to disrupt drug seeking remains unknown. In this study, we assessed the effect of JMV2959 on morphine induced memory reconsolidation to inhibit drug seeking after drug withdrawal. Our results showed that the administration of JMV2959 (6 mg/kg) significantly reduced environmental cue induced CPP, which suggested a preventive effect of JMV2959 on morphine induced memory reconsolidation. Additionally, JMV2959 administration significantly altered the locomotor activity and food and water intake but did not significantly alter the natural reward preference. We concluded that JMV2959 may be an effective candidate to treat drug addiction.

Assessing the role of ghrelin and the enzyme ghrelin O-acyltransferase (GOAT) system in food reward, food motivation, and binge eating behavior

The peripheral peptide hormone ghrelin is a powerful stimulator of food intake, which leads to body weight gain and adiposity in both rodents and humans. The hormone, thus, increases the vulnerability to obesity and binge eating behavior. Several studies have revealed that ghrelin's functions are due to its interaction with the growth hormone secretagogue receptor type 1a (GHSR1a) in the hypothalamic area; besides, ghrelin also promotes the reinforcing properties of hedonic food, acting at extra-hypothalamic sites and interacting with dopaminergic, cannabinoid, opioid, and orexin signaling. The hormone is primarily present in two forms in the plasma and the enzyme ghrelin O-acyltransferase (GOAT) allows the acylation reaction which causes the transformation of des-acyl-ghrelin (DAG) to the active form acyl-ghrelin (AG). DAG has been demonstrated to show antagonist properties; it is metabolically active, and counteracts the effects of AG on glucose metabolism and lipolysis, and reduces food consumption, body weight, and hedonic feeding response. Both peptides seem to influence the hypothalamic-pituitary-adrenal (HPA) axis and the corticosterone/cortisol level that drive the urge to eat under stressful conditions. These findings suggest that DAG and inhibition of GOAT may be targets for obesity and bingeing-related eating disorders and that AG/DAG ratio may be an important potential biomarker to assess the risk of developing maladaptive eating behaviors.

Levo-tetrahydropalmatine attenuates the acquisition of fentanyl-induced conditioned place preference and the changes in ERK and CREB phosphorylation expression in mice

Levo-tetrahydropalmatine (L-THP) is the main active ingredient of Corydalis and Stephania and is widely used for its sedative, analgesic, and neuroleptic effects. Though L-THP is an antagonist of dopamine receptors and has been proven to be effective in treating drug addiction, its effect on fentanyl-induced reward learning still remains unclear. This experiment was designed to investigate the effects of L-THP on fentanyl-induced rewarding behavior through conditioned place preference (CPP) in mice. Western blot assays were used to dissect the accompanying changes in the phosphorylation of extracellular signal-regulated kinase (ERK) and cAMP response element binding protein (CREB) in related brain regions, including the hippocampus (Hip), caudate putamen (CPu), prefrontal cortex (PFC), and nucleus accumbens (NAc), which may mediate the effects of L-THP on fentanyl-induced CPP. The results revealed that fentanyl could induce CPP in mice at doses of 0.025 mg/kg, 0.05 mg/kg, 0.1 mg/kg, and 0.2 mg/kg, and L-THP could attenuate the acquisition of fentany-induced CPP at a dose of 10.0 mg/kg. The levels of p-ERK and p-CREB of the saline+fentanyl group (0.05 mg/kg) increased significantly in the Hip, NAc, and PFC compared to the saline+saline group. Furthermore, L-THP (10.0 mg/kg) co-administered with fentanyl during conditioning prevented the enhanced phosphorylation of ERK and CREB in the Hip, NAc, and PFC. Our research revealed that L-THP could suppress the rewarding properties of fentanyl-induced CPP, the inhibitory effect may be related to the suppression of ERK and CREB phosphorylation in the Hip, NAc, and PFC of mice. Thus, L-THP may have therapeutic potential for fentanyl addiction.

Contribution of growth hormone secretagogue receptor (GHSR) signaling in the ventral tegmental area (VTA) to the regulation of social motivation in male mice

Most psychiatric disorders are characterized by deficits in the ability to interact socially with others. Ghrelin, a hormone normally associated with the regulation of glucose utilization and appetite, is also implicated in the modulation of motivated behaviors including those associated with food and sex rewards. Here we hypothesized that deficits in ghrelin receptor (growth hormone secretagogue receptor; GHSR) signaling are also associated with deficits in social motivation in male mice. To test this hypothesis, we compared social motivation in male mice lacking GHSR or mice treated with the GHSR antagonist JMV2959 with that of WT or vehicle-treated mice. GHSR signaling in dopamine cells of the ventral tegmental area (VTA) has been implicated in the control of sexual behavior, thus we further hypothesized that GHSR signaling in the VTA is important for social motivation. Thus, we conducted studies where we delivered JMV2959 to block GHSR in the VTA of mice, and studies where we rescued the expression of GHSR in the VTA of GHSR knockout (KO) mice. Mice lacking GHSR or injected with JMV2959 peripherally for 3 consecutive days displayed lower social motivation as reflected by a longer latency to approach a novel conspecific and shorter interaction time compared to WT or vehicle-treated controls. Furthermore, intra-VTA infusion of JMV2959 resulted in longer latencies to approach a novel conspecific, whereas GHSR KO mice with partial rescue of the GHSR showed decreased latencies to begin a novel social interaction. Together, these data suggest that GHSR in the VTA facilitate social approach in male mice, and GHSR-signaling deficits within the VTA result in reduced motivation to interact socially.

Converging vulnerability factors for compulsive food and drug use

Highly palatable foods and substance of abuse have intersecting neurobiological, metabolic and behavioral effects relevant for understanding vulnerability to conditions related to food (e.g., obesity, binge eating disorder) and drug (e.g., substance use disorder) misuse. Here, we review data from animal models, clinical populations and epidemiological evidence in behavioral, genetic, pathophysiologic and therapeutic domains. Results suggest that consumption of highly palatable food and drugs of abuse both impact and conversely are regulated by metabolic hormones and metabolic status. Palatable foods high in fat and/or sugar can elicit adaptation in brain reward and withdrawal circuitry akin to substances of abuse. Intake of or withdrawal from palatable food can impact behavioral sensitivity to drugs of abuse and vice versa. A robust literature suggests common substrates and roles for negative reinforcement, negative affect, negative urgency, and impulse control deficits, with both highly palatable foods and substances of abuse. Candidate genetic risk loci shared by obesity and alcohol use disorders have been identified in molecules classically associated with both metabolic and motivational functions. Finally, certain drugs may have overlapping therapeutic potential to treat obesity, diabetes, binge-related eating disorders and substance use disorders. Taken together, data are consistent with the hypotheses that compulsive food and substance use share overlapping, interacting substrates at neurobiological and metabolic levels and that motivated behavior associated with feeding or substance use might constitute vulnerability factors for one another. This article is part of the special issue on 'Vulnerabilities to Substance Abuse'.

Aversion-resistant fentanyl self-administration in mice

RATIONALE

Animal models of compulsive drug use that continues despite negative consequences can be used to investigate the neural mechanisms of addiction. However, models of punished or aversion-resistant opioid self-administration are notably lacking.

OBJECTIVES

We sought to develop an aversion-resistant, oral fentanyl self-administration paradigm.

METHODS

In Experiment 1, C57BL/6J male and female, adult mice consumed fentanyl (10 μg/mL) in a two-bottle drinking in the dark task and escalating concentrations of quinine were added to the bottles. In Experiment 2, mice were trained to administer oral fentanyl (10 μg/mL) in an operant response task. Quinine was next added to the fentanyl solution in escalating concentrations. In Experiment 3, mice were trained to respond for oral fentanyl or fentanyl adulterated with 500 μM quinine on every session. In Experiment 4, mice were trained to respond for a 1% sucrose solution before introduction of quinine.

RESULTS

Quinine reduced two-bottle choice consumption in males but not in females. Both sexes demonstrated the ability to detect the selected concentrations of quinine in fentanyl. In the operant chamber, mice responded robustly for oral fentanyl but introduction of quinine at any stage of training was insufficient to reduce responding. In contrast, quinine reduced responding for sucrose at concentrations above 250 μM.

CONCLUSIONS

Mice will respond for and consume oral fentanyl in both a two-bottle choice and an operant response task. Quinine is detectable in fentanyl but mice will continue to respond for and consume fentanyl with quinine in both paradigms. These data support the use of these models in behavioral studies of compulsive-like opioid use.

Cannabinoid-Induced Conditioned Place Preference, Intravenous Self-Administration, and Behavioral Stimulation Influenced by Ghrelin Receptor Antagonism in Rats

Cannabis/cannabinoids are widely used for recreational and therapy purposes, but their risks are largely disregarded. However, cannabinoid-associated use disorders and dependence are alarmingly increasing and an effective treatment is lacking. Recently, the growth hormone secretagogue receptor (GHSR1A) antagonism was proposed as a promising mechanism for drug addiction therapy. However, the role of GHS-R1A and its endogenous ligand ghrelin in cannabinoid abuse remains unclear. Therefore, the aim of our study was to investigate whether the GHS-R1A antagonist JMV2959 could reduce the tetrahydrocannabinol (THC)-induced conditioned place preference (CPP) and behavioral stimulation, the WIN55,212-2 intravenous self-administration (IVSA), and the tendency to relapse. Following an ongoing WIN55,212-2 self-administration, JMV2959 3 mg/kg was administered intraperitoneally 20 min before three consequent daily 120-min IVSA sessions under a fixed ratio FR1, which significantly reduced the number of the active lever-pressing, the number of infusions, and the cannabinoid intake. Pretreatment with JMV2959 suggested reduction of the WIN55,212-2-seeking/relapse-like behavior tested in rats on the twelfth day of the forced abstinence period. On the contrary, pretreatment with ghrelin significantly increased the cannabinoid IVSA as well as enhanced the relapse-like behavior. Co-administration of ghrelin with JMV2959 abolished/reduced the significant efficacy of the GHS-R1A antagonist in the cannabinoid IVSA. Pretreatment with JMV2959 significantly and dose-dependently reduced the manifestation of THC-induced CPP. The THC-CPP development was reduced after the simultaneous administration of JMV2959 with THC during conditioning. JMV2959 also significantly reduced the THC-induced behavioral stimulation in the LABORAS cage. Our findings suggest that GHS-R1A importantly participates in the rewarding/reinforcing effects of cannabinoids.

Alterations in Rat Accumbens Dopamine, Endocannabinoids and GABA Content During WIN55,212-2 Treatment: The Role of Ghrelin

The endocannabinoid/CB1R system as well as the central ghrelin signalling with its growth hormone secretagogoue receptors (GHS-R1A) are importantly involved in food intake and reward/reinforcement processing and show distinct overlaps in distribution within the relevant brain regions including the hypothalamus (food intake), the ventral tegmental area (VTA) and the nucleus accumbens (NAC) (reward/reinforcement). The significant mutual interaction between these systems in food intake has been documented; however, the possible role of ghrelin/GHS-R1A in the cannabinoid reinforcement effects and addiction remain unclear. Therefore, the principal aim of the present study was to investigate whether pretreatment with GHS-R1A antagonist/JMV2959 could reduce the CB1R agonist/WIN55,212-2–induced dopamine efflux in the nucleus accumbens shell (NACSh), which is considered a crucial trigger impulse of the addiction process. The synthetic aminoalklylindol cannabinoid WIN55,212-2 administration into the posterior VTA induced significant accumbens dopamine release, which was significantly reduced by the 3 mg/kg i.p. JMV2959 pretreatment. Simultaneously, the cannabinoid-increased accumbens dopamine metabolic turnover was significantly augmented by the JMV2959 pretreament. The intracerebral WIN55,212-2 administration also increased the endocannabinoid arachidonoylethanolamide/anandamide and the 2-arachidonoylglycerol/2-AG extracellular levels in the NACSh, which was moderately but significantly attenuated by the JMV2959 pretreatment. Moreover, the cannabinoid-induced decrease in accumbens γ-aminobutyric acid/gamma-aminobutyric acid levels was reversed by the JMV2959 pretreatment. The behavioural study in the LABORAS cage showed that 3 mg/kg JMV2959 pretreatment also significantly reduced the systemic WIN55,212-2-induced behavioural stimulation. Our results demonstrate that the ghrelin/GHS-R1A system significantly participates in the rewarding/reinforcing effects of the cannabinoid/CB1 agonist that are involved in cannabinoid addiction processing.

An overview of ghrelin O-acyltransferase inhibitors: a literature and patent review for 2010-2019

INTRODUCTION

The peptide hormone ghrelin regulates physiological processes associated with energy homeostasis such as appetite, insulin signaling, glucose metabolism, and adiposity. Ghrelin has also been implicated in a growing number of neurological pathways involved in stress response and addiction behavior. For ghrelin to bind the growth hormone secretagogue receptor 1a (GHS-R1a) and activate signaling, the hormone must first be octanoylated on a specific serine side chain. This key transformation is performed by the enzyme ghrelin O-acyltransferase (GOAT), and therefore GOAT inhibitors may be useful in treating disorders related to ghrelin signaling such as diabetes, obesity, and related metabolic syndromes.

AREAS COVERED

This report covers ghrelin and GOAT as potential therapeutic targets and summarizes work on GOAT inhibitors through the end of 2019, highlighting recent successes with both peptidomimetics and small molecule GOAT inhibitors as potent modulators of GOAT-catalyzed ghrelin octanoylation.

EXPERT OPINION

A growing body of biochemical and structural knowledge regarding the ghrelin/GOAT system now enables multiple avenues for identifying and optimizing GOAT inhibitors. We are at the beginning of a new era with increased opportunities for leveraging ghrelin and GOAT in the understanding and treatment of multiple health conditions including diabetes, obesity, and addiction.