Ghrelin activates the mesolimbic dopamine system via nitric oxide associated mechanisms in the ventral tegmental area.

Besides enhanced feeding, the orexigenic peptide ghrelin activates the mesolimbic dopamine system to cause reward as measured by locomotor stimulation, dopamine release in nucleus accumbens shell (NAcS), and conditioned place preference. Although the ventral tegmental area (VTA) appears to be a central brain region for this ghrelin-reward, the underlying mechanisms within this area are unknown. The findings that the gaseous neurotransmitter nitric oxide (NO) modulate the ghrelin enhanced feeding, led us to hypothesize that ghrelin increases NO levels in the VTA, and thereby stimulates reward-related behaviors. We initially demonstrated that inhibition of NO synthesis blocked the ghrelin-induced activation of the mesolimbic dopamine system. We then established that antagonism of downstream signaling of NO in the VTA, namely sGC, prevents the ability of ghrelin to stimulate the mesolimbic dopamine system. The association of ghrelin to NO was further strengthened by in vivo electrochemical recordings showing that ghrelin enhances the NO release in the VTA. Besides a GABAB -receptor agonist, known to reduce NO and cGMP, blocks the stimulatory properties of ghrelin. The present series of experiments reveal that ablated NO signaling, through pharmacologically inhibiting the production of NO and/or cGMP, prevents the ability of ghrelin to induced reward-related behaviors.

Brain region-specific neuromedin U signalling regulates alcohol-related behaviours and food intake in rodents.

Albeit neuromedin U (NMU) attenuates alcohol-mediated behaviours, its mechanisms of action are poorly defined. Providing that the behavioural effects of alcohol are processed within the nucleus accumbens (NAc) shell, anterior ventral tegmental area (aVTA), and laterodorsal tegmental area (LDTg), we assessed the involvement of NMU signalling in the aforementioned areas on alcohol-mediated behaviours in rodents. We further examined the expression of NMU and NMU receptor 2 (NMUR2) in NAc and the dorsal striatum of high compared with low alcohol-consuming rats, as this area is of importance in the maintenance of alcohol use disorder (AUD). Finally, we investigated the involvement of NAc shell, aVTA and LDTg in the consumption of chow and palatable peanut butter, to expand the link between NMU and reward-related behaviours. We demonstrated here, that NMU into the NAc shell, but not aVTA or LDTg, blocked the ability of acute alcohol to cause locomotor stimulation and to induce memory retrieval of alcohol reward, as well as reduced peanut butter in mice. In addition, NMU into NAc shell decreased alcohol intake in rats. On a molecular level, we found increased NMU and decreased NMUR2 expression in the dorsal striatum in high compared with low alcohol-consuming rats. Both aVTA and LDTg, rather than NAc shell, were identified as novel sites of action for NMU's anorexigenic properties in mice based on NMU's ability to selectively reduce chow intake when injected to these areas. Collectively, these data indicate that NMU signalling in different brain areas selectively modulates different behaviours.

Activation of amylin receptors attenuates alcohol-mediated behaviours in rodents.

Alcohol expresses its reinforcing properties by activating areas of the mesolimbic dopamine system, which consists of dopaminergic neurons projecting from the ventral tegmental area to the nucleus accumbens. The findings that reward induced by food and addictive drugs involve common mechanisms raise the possibility that gut-brain hormones, which control appetite, such as amylin, could be involved in reward regulation. Amylin decreases food intake, and despite its implication in the regulation of natural rewards, tenuous evidence support amylinergic mediation of artificial rewards, such as alcohol. Therefore, the present experiments were designed to investigate the effect of salmon calcitonin (sCT), an amylin receptor agonist and analogue of endogenous amylin, on various alcohol-related behaviours in rodents. We showed that acute sCT administration attenuated the established effects of alcohol on the mesolimbic dopamine system, particularly alcohol-induced locomotor stimulation and accumbal dopamine release. Using the conditioned place preference model, we demonstrated that repeated sCT administration prevented the expression of alcohol's rewarding properties and that acute sCT administration blocked the reward-dependent memory consolidation. In addition, sCT pre-treatment attenuated alcohol intake in low alcohol-consuming rats, with a more evident decrease in high alcohol consumers in the intermittent alcohol access model. Lastly, sCT did not alter peanut butter intake, blood alcohol concentration and plasma corticosterone levels in mice. Taken together, the present data support that amylin signalling is involved in the expression of alcohol reinforcement and that amylin receptor agonists could be considered for the treatment of alcohol use disorder in humans.

Central administration of the anorexigenic peptide neuromedin U decreases alcohol intake and attenuates alcohol-induced reward in rodents.

By investigating the neurochemical mechanisms through which alcohol activates the brain reward systems, novel treatment strategies for alcohol use disorder (AUD), a chronic relapsing disease, can be developed. In contrast to the common view of the function of gut-brain peptides, such as neuromedin U (NMU), to regulate food intake and appetite, a novel role in reinforcement mediation has been implied. The anorexigenic effects of NMU are mediated via NMU2 receptors, preferably in the arcuate nucleus and paraventricular nucleus. The expression of NMU2 receptors is also expressed in several reward-related areas in the brain, suggesting a role in reward regulation. The present experiments were therefore set up to investigate the effect of intracerebroventricular administration of NMU on alcohol-mediated behaviors in rodents. We found that central administration of NMU attenuated alcohol-induced locomotor stimulation, accumbal dopamine release and the expression of conditioned place preference in mice. In addition, NMU dose dependently decreased alcohol intake in high, but not in low, alcohol-consuming rats. Central NMU administration did not alter the blood alcohol concentrations nor change the corticosterone levels in rodents. Given that AUD is a major health-care challenge causing an enormous cost to society and novel treatment strategies are warranted, our data suggest that NMU analogues deserve to be evaluated as novel treatment of AUD in humans.

The glucagon-like peptide 1 receptor agonist liraglutide attenuates the reinforcing properties of alcohol in rodents.

The incretin hormone, glucagon-like peptide 1 (GLP-1), regulates gastric emptying, glucose-dependent stimulation of insulin secretion and glucagon release, and GLP-1 analogs are therefore approved for treatment of type II diabetes. GLP-1 receptors are expressed in reward-related areas such as the ventral tegmental area and nucleus accumbens, and GLP-1 was recently shown to regulate several alcohol-mediated behaviors as well as amphetamine-induced, cocaine-induced and nicotine-induced reward. The present series of experiments were undertaken to investigate the effect of the GLP-1 receptor agonist, liraglutide, on several alcohol-related behaviors in rats that model different aspects of alcohol use disorder in humans. Acute liraglutide treatment suppressed the well-documented effects of alcohol on the mesolimbic dopamine system, namely alcohol-induced accumbal dopamine release and conditioned place preference in mice. In addition, acute administration of liraglutide prevented the alcohol deprivation effect and reduced alcohol intake in outbred rats, while repeated treatment of liraglutide decreased alcohol intake in outbred rats as well as reduced operant self-administration of alcohol in selectively bred Sardinian alcohol-preferring rats. Collectively, these data suggest that GLP-1 receptor agonists could be tested for treatment of alcohol dependence in humans.

Semaglutide reduces alcohol intake and relapse-like drinking in male and female rats.

BACKGROUND: Glucagon-like peptide1 receptor (GLP-1R) agonists have been found to reduce alcohol drinking in rodents and overweight patients with alcohol use disorder (AUD). However, the probability of low semaglutide doses, an agonist with higher potency and affinity for GLP-1R, to attenuate alcohol-related responses in rodents and the underlying neuronal mechanisms is unknown. METHODS: In the intermittent access model, we examined the ability of semaglutide to decrease alcohol intake and block relapse-like drinking, as well as imaging the binding of fluorescently marked semaglutide to nucleus accumbens (NAc) in both male and female rats. The suppressive effect of semaglutide on alcohol-induced locomotor stimulation and in vivo dopamine release in NAc was tested in male mice. We evaluated effect of semaglutide on the in vivo release of dopamine metabolites (DOPAC and HVA) and gene expression of enzymes metabolising dopamine (MAOA and COMT) in male mice. FINDINGS: In male and female rats, acute and repeated semaglutide administration reduced alcohol intake and prevented relapse-like drinking. Moreover, fluorescently labelled semaglutide was detected in NAc of alcohol-drinking male and female rats. Further, semaglutide attenuated the ability of alcohol to cause hyperlocomotion and to elevate dopamine in NAc in male mice. As further shown in male mice, semaglutide enhanced DOPAC and HVA in NAc when alcohol was onboard and increased the gene expression of COMT and MAOA. INTERPRETATION: Altogether, this indicates that semaglutide reduces alcohol drinking behaviours, possibly via a reduction in alcohol-induced reward and NAc dependent mechanisms. As semaglutide also decreased body weight of alcohol-drinking rats of both sexes, upcoming clinical studies should test the plausibility that semaglutide reduces alcohol intake and body weight in overweight AUD patients. FUNDING: Swedish Research Council (2019-01676), LUA/ALF (723941) from the Sahlgrenska University Hospital and the Swedish brain foundation.

An amylin and calcitonin receptor agonist modulates alcohol behaviors by acting on reward-related areas in the brain.

Alcohol causes stimulatory behavioral responses by activating reward-processing brain areas including the laterodorsal (LDTg) and ventral tegmental areas (VTA) and the nucleus accumbens (NAc). Systemic administration of the amylin and calcitonin receptor agonist salmon calcitonin (sCT) attenuates alcohol-mediated behaviors, but the brain sites involved in this process remain unknown. Firstly, to identify potential sCT sites of action in the brain, we used immunohistochemistry after systemic administration of fluorescent-labeled sCT. We then performed behavioral experiments to explore how infused sCT into the aforementioned reward-processing brain areas affects acute alcohol-induced behaviors in mice and chronic alcohol consumption in rats. We show that peripheral sCT crosses the blood brain barrier and is detected in all the brain areas studied herein. sCT infused into the LDTg attenuates alcohol-evoked dopamine release in the NAc shell in mice and reduces alcohol intake in rats. sCT into the VTA blocks alcohol-induced locomotor stimulation and dopamine release in the NAc shell in mice and decreases alcohol intake in rats. Lastly, sCT into the NAc shell prevents alcohol-induced locomotor activity in mice. Our data suggest that central sCT modulates the ability of alcohol to activate reward-processing brain regions.

Long-term treatment with a glucagon-like peptide-1 receptor agonist reduces ethanol intake in male and female rats.

Given the limited efficacy of available pharmacotherapies for treatment of alcohol use disorder (AUD), the need for new medications is substantial. Preclinical studies have shown that acute administration of glucagon-like peptide-1 receptor (GLP-1R) agonists inhibits various ethanol-related behaviours, indicating this system as a potential target for AUD. However, the effects of long-term systemic treatment of GLP-1R agonists on ethanol intake in male and female rodents are to date unknown. Therefore, we investigated the effects of 9 or 5 weeks of once weekly administration of dulaglutide, a long-acting GLP-1R agonist, on ethanol intake in male and female rats. The ethanol intake during treatment discontinuation was also monitored. In an initial attempt to identify preliminary underlying mechanisms, the effects of 9 weeks of once weekly dulaglutide treatment on monoaminergic signalling in reward-related areas were explored in both sexes. We found that 9 or 5 weeks of once weekly dulaglutide treatment reduced ethanol intake and preference in male and female rats. Following discontinuation of dulaglutide treatment, the decrease in ethanol consumption was prolonged in males, but not females. We demonstrated that 9 weeks of dulaglutide treatment differentially influenced monoaminergic signalling in reward-related areas of male and female rats. Collectively, these data imply that the GLP-1R attracts interest as a potential molecular target in the medical treatment of AUD in humans: more specifically, dulaglutide should be evaluated as a potential medication for treatment thereof.

Brain region specific glucagon-like peptide-1 receptors regulate alcohol-induced behaviors in rodents.

Glucagon-like peptide 1 (GLP-1), an incretin hormone that reduces food intake, was recently established as a novel regulator of alcohol-mediated behaviors. Clinically available analogues pass freely into the brain, but the mechanisms underlying GLP-1-modulated alcohol reward remains largely unclear. GLP-1 receptors (GLP-1R) are expressed throughout the nuclei of importance for acute and chronic effects of alcohol, such as the laterodorsal tegmental area (LDTg), the ventral tegmental area (VTA) and the nucleus accumbens (NAc). We therefore evaluated the effects of bilateral infusion of the GLP-1R agonist exendin-4 (Ex4) into NAc shell, anterior (aVTA), posterior (pVTA) or LDTg on the acute alcohol-induced locomotor stimulation and memory of alcohol reward in the conditioned place preference (CPP) model in mice, as well as on alcohol intake in rats consuming high amounts of alcohol for 12 weeks. Ex4 into the NAc shell blocks alcohol-induced locomotor stimulation and memory of alcohol reward as well as decreases alcohol intake. The GLP-1R expression in NAc is elevated in high compared to low alcohol-consuming rats. On the contrary, GLP-1R activation in the aVTA does not modulate alcohol-induced behaviors. Ex4 into the pVTA prevents alcohol-induced locomotor simulation, but does neither modulate CPP-dependent alcohol memory nor alcohol intake. Intra-LDTg-Ex4 attenuates alcohol-induced locomotor stimulation and reduces alcohol intake, but does not affect memory of alcohol reward. Collectively, these data provide additional knowledge of the functional role of GLP-1R in reward-related areas for alcohol-mediated behaviors and further support GLP-1R as a potential treatment target for alcohol use disorder.

Glucagon-like peptide-1 receptors within the nucleus of the solitary tract regulate alcohol-mediated behaviors in rodents.

The ability of glucagon-like peptide-1 (GLP-1) to reduce food intake involves activation of GLP-1 receptors (GLP-1R) in the nucleus of the solitary tract (NTS). It has also been demonstrated that systemic administration of GLP-1R agonists attenuates alcohol-mediated behaviors via, to date, unknown mechanisms. Therefore, we evaluated the effects of NTS-GLP-1R activation by exendin-4 (Ex4) on alcohol-induced locomotor stimulation, accumbal dopamine release and memory of alcohol reward in the conditioned place preference (CPP) model in mice. Moreover, the ability of Ex4 infusion into the NTS on alcohol intake was explored in rats. Ex4 into the NTS inhibits the acute effects of alcohol as measured by alcohol-induced locomotor stimulation, accumbal dopamine release and the memory consolidation of alcohol reward in the CPP paradigm. In addition, NTS-Ex4 dose-dependently decreases alcohol intake in rats consuming alcohol for 12 weeks. Pharmacological suppression of GLP-1R in the NTS prevents the ability of systemic Ex4 to block the alcohol-induced locomotor stimulation in mice. These data add a functional role of GLP-1R within the NTS, involving alcohol-related behaviors. In addition, they may provide insight into the GLP-1R containing brain areas that modulate the ability of GLP-1R agonists to reduce alcohol reinforcement. Collectively, this further supports GLP-1R as potential treatment targets for alcohol use disorder.

An amylin analogue attenuates alcohol-related behaviours in various animal models of alcohol use disorder.

Recent findings have identified salmon calcitonin (sCT), an amylin receptor agonist and analogue of endogenous amylin, as a potential regulator of alcohol-induced activation of the mesolimbic dopamine system and alcohol consumption. Providing that the role of amylin signalling in alcohol-related behaviours remains unknown, the present experiments investigate the effect of sCT on these behaviours and the mechanisms involved. We showed that repeated sCT administration decreased alcohol and food intake in outbred rats. Moreover, single administration of the potent amylin receptor antagonist, AC187, increased short-term alcohol intake in outbred alcohol-consuming rats, but did not affect food intake. Acute administration of sCT prevented relapse-like drinking in the "alcohol deprivation effect" model in outbred alcohol-experienced rats. Additionally, acute sCT administration reduced operant oral alcohol self-administration (under the fixed ratio 4 schedule of reinforcement) in selectively bred Sardinian alcohol-preferring rats, while it did not alter operant self-administration (under the progressive ratio schedule of reinforcement) of a highly palatable chocolate-flavoured beverage in outbred rats. Lastly, we identified differential amylin receptor expression in high compared to low alcohol-consuming rats, as reflected by decreased calcitonin receptor and increased receptor activity modifying protein 1 expression in the nucleus accumbens (NAc) of high consumers. Collectively, our data suggest that amylin signalling, especially in the NAc, may contribute to reduction of various alcohol-related behaviours.

A cannabinoid receptor antagonist attenuates ghrelin-induced activation of the mesolimbic dopamine system in mice.

Ghrelin has been attributed various physiological processes including food intake and reward regulation, through activation of the mesolimbic dopamine system. Reward modulation involves the mesolimbic dopamine system, consisting of the ventral tegmental area (VTA) dopamine neurons targeting nucleus accumbens (NAc), a system that ghrelin activates through VTA-dependent mechanisms. In the first study, we found that systemic intraperitoneal (ip) administration of rimonabant attenuated intracerebroventricular (icv) ghrelin's ability to cause locomotor stimulation and NAc dopamine release in mice. Ghrelin-induced (icv) chow intake was not altered by rimonabant administration (ip). Finally, we showed that bilateral VTA administration of rimonabant blocks the ability of intra-VTA administered ghrelin to increase locomotor activity, but does not affect food intake in mice. Collectively, these data indicate clear dissociation between regulation of food intake and activation of the mesolimbic dopamine system.

The Anorexigenic Peptide Neuromedin U (NMU) Attenuates Amphetamine-Induced Locomotor Stimulation, Accumbal Dopamine Release and Expression of Conditioned Place Preference in Mice.

Amphetamine dependence, besides its substantial economical consequence, is a serious cause of mortality and morbidity. By investigations of the neurochemical correlates through which addictive drugs, such as amphetamine, activate the mesoaccumbal dopamine system unique targets for treatment of drug addiction can be identified. This reward link consists of a dopamine projection from the ventral tegmental area to the nucleus accumbens (NAc) suggesting that these brain areas are important for reward. The physiological function of gut-brain peptides has expanded beyond food intake modulation and involves regulation of drug reinforcement. A novel candidate for reward regulation is the anorexigenic peptide neuromedin U (NMU). We therefore investigated the effects of intracerebroventricular (icv) administration of NMU on amphetamine's well-documented effects on the mesoaccumbal dopamine system, i.e. locomotor stimulation and accumbal dopamine release in mice. In addition, the effect of accumbal NMU administration on locomotor activity was examined. The effect of NMU, icv or intra-NAc, on the expression of conditioned place preference (CPP) was elucidated. Firstly, we showed that icv administration of NMU attenuate the amphetamine-induced locomotor stimulation, accumbal dopamine release and expression of CPP in mice. Secondly, we found that a lower dose of NMU (icv) reduce the amphetamine-induced locomotor stimulation in mice. Thirdly, we demonstrated that NMU administration into the NAc block the ability of amphetamine to cause a locomotor stimulation in mice. However, accumbal NMU administration did not attenuate the amphetamine-induced expression of CPP in mice. Our novel data suggest that central NMU signalling is involved in development of amphetamine dependence.