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Gut-brain hormones such as ghrelin have recently been suggested to have a role in reward regulation. Ghrelin was traditionally known to regulate food intake and body weight homoeostasis. In addition, recent work has pin-pointed that this peptide has a novel role in drug-induced reward, including morphine-induced increase in the extracellular levels of accumbal dopamine in rats. Herein the effect of the ghrelin receptor (GHS-R1A) antagonist, JMV2959, on morphine-induced activation of the mesolimbic dopamine system was investigated in mice. In addition, the effects of JMV2959 administration on opioid peptide levels in reward related areas were investigated. In the present series of experiment we showed that peripheral JMV2959 administration, at a dose with no effect per se, attenuates the ability of morphine to cause locomotor stimulation, increase the extracellular levels of accumbal dopamine and to condition a place preference in mice. JMV2959 administration significantly increased tissue levels of Met-enkephalin-Arg6Phe7 in the ventral tegmental area, dynorphin B in hippocampus and Leu-enkephalin-Arg6 in striatum. We therefore hypothesise that JMV2959 prevents morphine-induced reward via stimulation of delta receptor active peptides in striatum and ventral tegmental areas. In addition, hippocampal peptides that activate kappa receptor may be involved in JMV2959׳s ability to regulate memory formation of reward. Given that development of drug addiction depends, at least in part, of the effects of addictive drugs on the mesolimbic dopamine system the present data suggest that GHS-R1A antagonists deserve to be elucidated as novel treatment strategies of opioid addiction.

Food intake and appetite are regulated by various circulating hormones including ghrelin and glucagon-like-peptide 1 (GLP-1). Ghrelin, mainly released from the stomach, increases food intake, induces appetite, enhances adiposity as well as releases growth hormone. Hypothalamic “ghrelin receptors” (GHS-R1A) have a critical role in food intake regulation, but GHS-R1A are also expressed in reward related areas. GLP-1 is produced in the intestinal mucosa as well as in the hindbrain in response to nutrient ingestion. This gut-brain hormone reduces food intake as well as regulates glucose homeostasis, foremost via GLP-1 receptors in hypothalamus and brain stem. However, GLP-1 receptors are expressed in areas intimately associated with reward regulation. Given that regulation of food and drug intake share common neurobiological substrates, the possibility that ghrelin and GLP-1 play an important role in reward regulation should be considered. Indeed, this leading article describes that the orexigenic peptide ghrelin activates the cholinergic–dopaminergic reward link, an important part of the reward systems in the brain associated with reinforcement and thereby increases the incentive salience for motivated behaviors via this system. We also review the role of ghrelin signaling for reward induced by alcohol and addictive drugs from a preclinical, clinical and human genetic perspective. In addition, the recent findings showing that GLP-1 controls reward induced by alcohol, amphetamine, cocaine and nicotine in rodents are overviewed herein. Finally, the role of several other appetite regulatory hormones for reward and addiction is briefly discussed. Collectively, these data suggest that ghrelin and GLP-1 receptors may be novel targets for development of pharmacological treatments of alcohol and drug dependence.

Development of alcohol dependence, a chronic and relapsing disease, largely depends on the effects of alcohol on the brain reward systems, specifically the cholinergic‐dopaminergic reward link (for review, see Larsson and Engel, 2004; Soderpalm et al., 2009; Volkow and Li, 2004). There is a comorbidity between alcohol dependence and compulsive over eating (for review, see Dickson et al., 2011), indicating that gut‐brain hormones, such as ghrelin, could be common biological mechanisms important for reward induced by food and alcohol. Ghrelin, the endogenous ligand for the growth hormone secretagogue receptor 1A (GHS‐R1A), acts as an important regulator of energy balance (Kojima et al., 1999). Indeed, ghrelin induces adiposity and increases body weight in rats (Tschop et al., 2000). Additionally, central as well as peripheral ghrelin administration increases food intake in rodents, an effect predominantly mediated via hypothalamic GHS‐R1A (Wren et al., 2000, 2001b). Human studies show that ghrelin increases the sensation of hunger and appetite. (Wren et al., 2001a). The plasma levels of ghrelin rise preprandially and fall postprandially in humans as well as in rodents, implying that ghrelin induces meal initiation (for review, see Egecioglu et al., 2011). The findings that GHS‐R1A are expressed in reward nodes associated with the cholinergic dopaminergic reward link including the ventral tegmental area, the nucleus accumbens, and the laterodorsal tegmental area (for review, see Dickson et al., 2011) imply that ghrelin may be involved in reward regulation. Initially, it was shown that ghrelin activates cholinergic‐dopaminergic reward link (Abizaid et al., 2006; Jerlhag et al., 2006a, 2007). This was corroborated by the finding showing that the rewarding properties of alcohol, as measured by locomotor stimulation, accumbal dopamine release, and conditioned place preference are attenuated in mice with suppressed GHS‐R1A and ghrelin signaling (Jerlhag et al., 2009, 2011) and that GHS‐R1A antagonists reduced the intake and the motivation to consume alcohol in rodents (Jerlhag et al., 2009; Kaur and Ryabinin, 2010; Landgren et al., 2012). Supportively, human genetic findings show that a single‐nucleotide polymorphism in the GHS‐R1A gene is associated with high alcohol consumption in humans (Landgren et al., 2008). The findings that this gut‐brain hormone is produced centrally (Cowley et al., 2003; Lu et al., 2002; Mondal et al., 2005) as well as in the gastrointestinal tract (Kojima et al., 1999) raises the need for investigations regarding the importance of central versus peripheral ghrelin for alcohol reward. The present series of experiments was designed to investigate the role of circulating endogenous ghrelin, by using Spiegelmer NOX‐B11‐2, for alcohol‐induced reward, as measured by locomotor stimulation, accumbal dopamine release, and the expression of conditioned place preference in mice, as well as for alcohol intake in rats. NOX‐B11‐2 inhibits ghrelin's activation of GHS‐R1A‐expressing CHO cells by directly binding to the bioactive, acylated form of ghrelin (Shearman et al., 2006). NOX‐B11‐2 is expected to selectively neutralize ghrelin in the periphery since autoradiography studies of related compounds in rats and cynomolgus monkeys have shown that Spiegelmers do not enter the central nervous system. The results of the present experiments may be of clinical interest since prevention of ghrelin's brain access tentatively could be used as novel treatment of alcohol addiction.

SummaryDevelopment of alcohol use disorders largely depends on the effects of alcohol on the brain reward systems. Emerging evidence indicate that common mechanisms regulate food and alcohol intake and raise the possibility that endocrine signals from the gut may play an important role for alcohol consumption, alcohol-induced reward and the motivation to consume alcohol. Glucagon-like peptide 1 (GLP-1), a gastrointestinal peptide regulating food intake and glucose homeostasis, has recently been shown to target central brain areas involved in reward and motivation, including the ventral tegmental area and nucleus accumbens. Herein we investigated the effects of the GLP-1 receptor agonist, Exendin-4 (Ex4), on various measures of alcohol-induced reward as well as on alcohol intake and alcohol seeking behavior in rodents. Treatment with Ex4, at a dose with no effect per se, attenuated alcohol-induced locomotor stimulation and accumbal dopamine release in mice. Furthermore, conditioned place preference for alcohol was abolished by both acute and chronic treatment with Ex4 in mice. Finally we found that Ex4 treatment decreased alcohol intake, using the intermittent access 20% alcohol two-bottle-choice model, as well as alcohol seeking behavior, using the progressive ratio test in the operant self-administration model, in rats. These novel findings indicate that GLP-1 signaling attenuates the reinforcing properties of alcohol implying that the physiological role of GLP-1 extends beyond glucose homeostasis and food intake regulation. Collectively these findings implicate that the GLP-1 receptor may be a potential target for the development of novel treatment strategies for alcohol use disorders.

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.

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.

Background The orexigenic peptide ghrelin activates the reward systems, specifically the cholinergic–dopaminergic reward link, suggesting that ghrelin may increase the incentive salience of motivated behaviours such as food seeking. Moreover, central ghrelin signalling, involving the growth hormone secretagogue receptor 1A (GHS-R1A), is required for the rewarding properties, as measured by locomotor stimulation, accumbal dopamine release and conditioned place preference, of alcohol, cocaine as well as amphetamine. As the target circuits for other drugs of abuse, including nicotine, in the brain includes this reward link, we sought to determine whether the central ghrelin signalling system is involved in nicotine's activation of this system. Methods This was investigated by studying the effects of peripheral administration of a GHS-R1A antagonist (JMV2959) on the nicotine-induced locomotor simulation, accumbal dopamine release and conditioned place preference. Results In the present study we found that the ability of nicotine to increase the locomotor activity, accumbal dopamine release and to condition place preference were reduced in mice treated with a GHS-R1A antagonist. Conclusion Thus GHS-R1A appears to be required not only for alcohol, cocaine and amphetamine-induced reward, but also for reward induced by nicotine. Our data suggest that the central ghrelin signalling system may constitute a novel potential target for treatment of drug dependence.

Ghrelin, the first endogenous ligand for the type 1A growth hormone secretagogue receptor (GHS-R1A), plays a role in energy balance, feeding behavior, and reward. Previously, we showed that pharmacologic and genetic suppression of the GHS-R1A attenuates the alcohol-induced stimulation, accumbal dopamine release, and conditioned place preference as well as alcohol consumption in mice, implying that the GHS-R1A is required for alcohol reward. The present study further elucidates the role of ghrelin for alcohol-induced dopamine release in nucleus accumbens and locomotor stimulation by means of ghrelin knockout mice. We found that the ability of alcohol to increase accumbal dopamine release in wild-type mice is not observed in ghrelin knockout mice. Furthermore, alcohol induced a locomotor stimulation in the wild-type mice and ghrelin knockout mice; however, the locomotor stimulation in homozygote mice was significantly lower than in the wild-type mice. The present series of experiments suggest that endogenous ghrelin may be required for the ability of alcohol to activate the mesolimbic dopamine system.

Here we review the emerging neurobiological understanding of the role of the brain's reward system in the regulation of body weight in health and in disease. Common obesity is characterized by the over-consumption of palatable/rewarding foods, reflecting an imbalance in the relative importance of hedonic versus homeostatic signals. The popular 'incentive salience theory' of food reward recognises not only a hedonic/pleasure component ('liking') but also an incentive motivation component ('wanting' or 'reward-seeking'). Central to the neurobiology of the reward mechanism is the mesoaccumbal dopamine system that confers incentive motivation not only for natural rewards such as food but also by artificial rewards (eg. addictive drugs). Indeed, this mesoaccumbal dopamine system receives and integrates information about the incentive (rewarding) value of foods with information about metabolic status. Problematic over-eating likely reflects a changing balance in the control exerted by hypothalamic versus reward circuits and/or it could reflect an allostatic shift in the hedonic set point for food reward. Certainly, for obesity to prevail, metabolic satiety signals such as leptin and insulin fail to regain control of appetitive brain networks, including those involved in food reward. On the other hand, metabolic control could reflect increased signalling by the stomach-derived orexigenic hormone, ghrelin. We have shown that ghrelin activates the mesoaccumbal dopamine system and that central ghrelin signalling is required for reward from both chemical drugs (eg alcohol) and also from palatable food. Future therapies for problematic over-eating and obesity may include drugs that interfere with incentive motivation, such as ghrelin antagonists.

Here we sought to determine whether ghrelin's central effects on food intake can be interrupted by nicotine acetylcholine receptor (nAChR) blockade. Ghrelin regulates mesolimbic dopamine neurons projecting from the ventral tegmental area (VTA) to the nucleus accumbens, partly via cholinergic VTA afferents originating in the laterodorsal tegmental area (LDTg). Given that these cholinergic projections to the VTA have been implicated in natural as well as drug-induced reinforcement, we sought to investigate the role of cholinergic signaling in ghrelin-induced food intake as well as fasting-induced food intake, for which endogenous ghrelin has been implicated. We found that i.p. treatment with the non-selective centrally active nAChR antagonist, mecamylamine decreased fasting-induced food intake in both mice and rats. Moreover, central administration of mecamylamine decreased fasting-induced food intake in rats. I.c.v. ghrelin-induced food intake was suppressed by mecamylamine i.p. but not by hexamethonium i.p., a peripheral nAChR antagonist. Furthermore, mecamylamine i.p. blocked food intake following ghrelin injection into the VTA. Expression of the ghrelin receptor, the growth hormone secretagogue receptor 1A, was found to co-localize with choline acetyltransferase, a marker of cholinergic neurons, in the LDTg. Finally, mecamylamine treatment i.p. decreased the ability of palatable food to condition a place preference. These data suggest that ghrelin-induced food intake is partly mediated via nAChRs and that nicotinic blockade decreases the rewarding properties of food.

Introduction Recently we demonstrated that genetic or pharmacological suppression of the central ghrelin signaling system, involving the growth hormone secretagogue receptor 1A (GHS-R1A), lead to a reduced reward profile from alcohol. As the target circuits for ghrelin in the brain include a mesolimbic reward pathway that is intimately associated with reward-seeking behaviour, we sought to determine whether the central ghrelin signaling system is required for reward from drugs of abuse other than alcohol, namely cocaine or amphetamine. Results We found that amphetamine—as well as cocaine-induced locomotor stimulation and accumbal dopamine release were reduced in mice treated with a GHS-R1A antagonist. Moreover, the ability of these drugs to condition a place preference was also attenuated by the GHS-R1A antagonist. Conclusions Thus GHS-R1A appears to be required not only for alcohol-induced reward, but also for reward induced by psychostimulant drugs. Our data suggest that the central ghrelin signaling system constitutes a novel potential target for treatment of addictive behaviours such as drug dependence.

DL-Metatyrosine (200 mg/kg intraperitoneally) administration after peripheral DOPA decarboxylase inhibition caused a temporary reversal of reserpine-induced (5 mg/kg intraperitoneally) suppression of a conditioned avoidance response in the rat, both with and without pretreatment with a dopamine-β-hydroxylase inhibitor, bis-(4-methyl-1-homopiperazinylthiocarbonyl)-disulfide, (FLA-63, 10 mg/kg intraperitoneally); but the duration of the reversal was shorter after FLA-63. After the injection of DL-metatyrosine there was a rapid and marked accumulation of metatyramine and a smaller accumulation of the β-hydroxylated metabolite, metaoctopamine. FLA-63 pretreatment prevented the metaoctopamine accumulation. Some of the behavioural effects observed after DL-metatyrosine may be due to displacement of endogenous noradrenaline not depleted by reserpine. The increase in locomotor activity after DL-metatyrosine was not affected by the dopamine-β-hydroxylase inhibitor. The results provide further support for the hypothesis that dopamine is important for elementary motor functions (e.g. locomotor activity) whilst additional noradrenaline receptor stimulation is essential for more complex and integrated hebaviour (e.g. a conditioned avoidance response).

Female rats were given 16% ethanol solution as the sole liquid during the entire period of gestation. At birth the offspring was removed and reared by foster dams consuming normal tap water. The development of sensory motor behaviour and emotional reactions was delayed by 1-2 days in the prenatally ethanol exposed pups as assessed by tests on body righting, acoustic startle response, air righting, rearing and ultrasonic vocalization. In the open-field test the normally occurring behavioural difference between the sexes was not found in the prenatally ethanol exposed pups. Both sexes of the ethanol exposed pups behaved like the female controls suggesting deficient masculinization of the ethanol exposed male pups during foetal age. Biochemical analysis of the brains showed a decreased synthesis of serotonin and dopamine.

Ghrelin stimulates appetite, increases food intake and causes adiposity by mechanisms that include direct actions on the brain. Previously, we showed that intracerebroventricular administration of ghrelin has stimulatory and dopamine-enhancing properties. These effects of ghrelin are mediated via central nicotine receptors, suggesting that ghrelin can activate the acetylcholine-dopamine reward link. This reward link consists of cholinergic input from the laterodorsal tegmental area (LDTg) to the mesolimbic dopamine system that originates in the ventral tegmental area (VTA) and projects to the nucleus accumbens. Given that growth hormone secretagogue receptors (GHSR-1A) are expressed in the VTA and LDTg, brain areas involved in reward, the present series of experiments were undertaken to examine the hypothesis that these regions may mediate the stimulatory and dopamine-enhancing effects of ghrelin, by means of locomotor activity and in vivo microdialysis in freely moving mice. We found that local administration of ghrelin into the VTA (1 microg in 1 microl) induced an increase in locomotor activity and in the extracellular concentration of accumbal dopamine. In addition, local administration of ghrelin into the LDTg (1 microg in 1 microl) caused a locomotor stimulation and an increase in the extracellular levels of accumbal dopamine. Taken together, this indicates that ghrelin might, via activation of GHSR-1A in the VTA and LDTg, stimulate the acetylcholine-dopamine reward link, implicating that ghrelin is a part of the neurochemical overlap between the reward systems and those that regulate energy balance.

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.

Aims Ghrelin initially emerged as a gut-brain hormone controlling food intake, meal initiation and appetite mainly via hypothalamic circuits in both rodents and humans. The findings that ghrelin receptors (GHS-R1A) are expressed in reward-related areas, including the nucleus accumbens (NAc) and ventral tegmental area (VTA), suggest that ghrelin is a novel reward regulator. Indeed, ghrelin signalling mediates the rewarding and motivational properties of addictive drugs. In addition, daily co-administration of a GHS-R1A antagonist and various addictive drugs prevents the drug-induced locomotor sensitization in rats. Methods The present series of experiment were designed to evaluate the effect of repeated pharmacological GHS-R1A suppression on drug-induced locomotor stimulation in more detail. Results We showed that sub-chronic pre-treatment of the GHS-R1A antagonist, JMV2959, attenuated the ability of acute administration of alcohol as well as of amphetamine to stimulate locomotion. However, there was no effect of sub-chronic JMV2959 treatment on locomotor activity per se or on the expression of the GHS-R1A gene ( Ghsr ) in the VTA or the NAc compared with vehicle treatment. In addition, sub-chronic ghrelin treatment caused a locomotor sensitization. Conclusions While previous research has pinpointed ghrelin as an appetite regulator the present study together with previous studies suggest that ghrelin signalling modulates various reward-mediated behaviours in rodents. Collectively, this suggests that the GHS-R1A could be a key target for novel treatment strategies for addiction.

Rationale Schizophrenic-spectrum patients commonly display deficits in preattentive information processing as evidenced, for example, by disrupted prepulse inhibition (PPI), a measure of sensorimotor gating. Similar disruptions in PPI can be induced in rodents and primates by the psychotomimetic drug phencyclidine (PCP), a noncompetitive inhibitor of the NMDA receptor. Mounting evidence suggests that the hunger hormone ghrelin and its constitutively active receptor influences neuronal circuits involved in the regulation of mood and cognition. Objectives In the present series of experiments, we investigated the effects of ghrelin and the growth hormone secretagogue receptor (GHS-R1A) neutral antagonist, JMV 2959, on acoustic startle responses (ASR), PPI, and PCP-induced alterations in PPI. Results Intraperitoneal (i.p.) administration of ghrelin (0.033, 0.1, and 0.33 mg/kg) did not alter the ASR or PPI in rats. Conversely, i.p. injection of JMV 2959 (1, 3, and 6 mg/kg), dose dependently decreased the ASR and increased PPI. Pretreatment with JMV 2959 at a dose with no effect on ASR or PPI per se, completely blocked PCP-induced (2 mg/kg) deficits in PPI while pretreatment with the highest dose of ghrelin did not potentiate or alter PPI responses of a sub-threshold dose of PCP (0.75 mg/kg). Conclusion These findings indicate that the GHS-R1A is involved in specific behavioral effects of PCP and may have relevance for patients with schizophrenia.

It is becoming increasingly apparent that there is a degree of neurochemical overlap between the reward systems and those regulating energy balance. We therefore investigated whether ghrelin, a stomach-derived and centrally derived orexigenic peptide, might act on the reward systems. Central ghrelin administration (1 microg/microL, to the third ventricle) induced an acute increase in locomotor activity as well as dopamine-overflow in the nucleus accumbens, suggesting that ghrelin can activate the mesoaccumbal dopamine system originating in the ventral tegmental area, a system associated with reward and motivated behaviour. The cholinergic afferents to the ventral tegmental area have been implicated in natural reward and in regulating mesoaccumbal dopamine neurons. The possibility that nicotinic receptors are involved in mediating the stimulatory and dopamine-enhancing effects of ghrelin is supported by the findings that peripheral injection of the unselective nicotinic antagonist mecamylamine (2.0 mg/kg) blocked these ghrelin-induced effects. Tentatively, ghrelin may, via activation of the acetylcholine-dopamine reward link, increase the incentive values of signals associated with motivated behaviours of importance for survival such as feeding behaviour. It will be important to discover whether this has therapeutic implications for compulsive addictive behaviours, such as eating behaviour disorders and drug dependence.

Growth hormone (GH) replacement in hypopituitary patients improves well-being and initiative. Experimental studies indicate that these psychic effects may be reflected in enhanced locomotor activity in mice. It is unknown whether these phenomena are mediated directly by GH or by circulating IGF-I. IGF-I production in the liver was inactivated at 6–10 wk of age (LI-IGF-I−/− mice), resulting in an 80–85% reduction of circulating IGF-I, and, secondary to this, increased GH secretion. Using activity boxes on three different occasions during 1 wk, 6-mo-old LI-IGF-I−/− mice had similar activity levels, and 14-mo-old mice had a moderate but significant decrease in activity level, compared with control mice. At 20 mo of age, the LI-IGF-I−/− mice displayed a more prominent decrease in activity level with decreased horizontal activity throughout the test period, and at day 1, there were several signs of an altered habituation process with different time patterns of locomotor activity and horizontal activity compared with the control mice. At days 3 and 5, rearing activity was lower in the 20-mo-old LI-IGF-I−/− mice. Anxiety level was unaffected in all age groups, as measured using the Montgomery’s elevated plus-maze. In conclusion, old LI-IGF-I−/− mice displayed a decrease in both horizontal and rearing (exploratory) activity level and an altered habituation process. These results indicate that liver-derived IGF-I mediates at least part of the effects of GH on exploratory activity in mice.

Concurrent use of ethanol and nicotine (tobacco) is often seen in human beings. In previous animal experiments, we have demonstrated that nicotinic acetylcholine receptors, especially α-conotoxin MII and mecamylamine sensitive receptors located in the ventral tegmental area may be involved in the stimulatory, dopamine enhancing, and rewarding effects of ethanol in rodents. Ethanol may exert these effects via direct interaction with nicotinic acetylcholine receptors and/or indirectly via enhancement of extracellular acetylcholine levels in the ventral tegmental area. The present experiments investigated a possible indirect effect of ethanol in stimulating the mesoaccumbal dopamine system. Methods: Neurochemical effects of voluntary ethanol intake on extracellular ventral tegmental acetylcholine and accumbal dopamine levels were measured by means of in vivo microdialysis with a two-probe approach in freely moving rats. Results: Obtained data indicate that voluntary ethanol intake (~0.7 g/kg/h) leads to an increase of extracellular acetylcholine levels in the ventral tegmental area, and an almost time-locked increase of dopamine levels in the nucleus accumbens. A positive correlation between the ventral tegmental acetylcholine levels and ethanol intake as well as preference was also observed. Conclusion: The present results suggest that voluntary ethanol intake enhances extracellular ventral tegmental acetylcholine that may interact with nicotinic acetylcholine receptors, possibly α-conotoxin MII sensitive receptors, localized in the ventral tegmental area that subsequently may stimulate dopamine overflow in the nucleus accumbens.

Latent inhibition (LI) is a behavioural procedure used to evaluate the potential propsychotic and antipsychotic properties of psychoactive drugs. In the present study, a conditioned taste aversion (CTA) procedure was used to investigate the effects of the nitric oxide (NO) synthase inhibitor, N G -nitro- l -arginine methyl ester (L-NAME), and the psychotomimetic drugs, phencyclidine (PCP) and d-amphetamine (d-AMP) on LI. PCP (2 mg/kg) and d-AMP (0.5 mg/kg) were both found to enhance LI in this procedure. The effect of d-AMP on LI was less pronounced and this drug also caused a weak disruption of taste aversion conditioning. Pretreatment with L -NAME (10 mg/kg) blocked the LI enhancing effect of PCP on LI but not that of d-AMP. L -NAME by itself caused an attenuation of LI. L -NAME has been shown to block also other behavioural and biochemical effects of PCP in previous studies and these results and the present findings suggest that at least some of the effects PCP are dependent on NO and possibly also that some NOS inhibitors may exert antipsychotic properties.

The most commonly abused drugs, alcohol and nicotine, are likely also the most costly drugs in terms of health and societal costs. A large body of evidence from epidemiological studies indicate that smoking and alcohol-intake are positively correlated. The mesocorticolimbic dopamine system has been implicated in mediating some of the reinforcing effects of ethanol, however, the mechanism(s) of action remains to be elucidated; consideration as to ethanol's ability to interact with ligand-gated ion channels should be considered. Accumulating evidence from electrophysiological, pharmacological and neurochemical studies suggest that ethanol may interact with the nicotinic acetylcholine receptor (nAChR). Thus, it has been shown that the ethanol-induced stimulation of the mesolimbic dopamine system and of locomotor activity as well as ethanol intake and preference in rodents may involve central nicotinic acetylcholine receptors. Additionally, data has been presented that nAChRs located in the ventral tegmental area may be of particular importance for these effects of ethanol. Studies aimed at defining the nAChR subpopulation(s) involved in mediating ethanol-induced locomotor stimulation and accumbal dopamine overflow as well as ethanol-intake have revealed that alpha(3)beta(2) or alpha(6) (using alpha-Conotoxin MII) but not alpha(4)beta(2) (using dihydro-beta-erythroidine) or alpha(7) (using methyllycaconitine), could represent targets for developing new drugs in the treatment of alcoholism. These results do not allow any conclusion as to whether the involvement nAChRs in mediating the effects of ethanol is direct and/or indirect. With regard to an indirect effect, evidence has accumulated indicating that the cholinergic excitatory input to the dopaminergic neurons in the ventral tegmental area may be an important part of the neuronal circuits mediating natural as well as drug-rewarded behavior. The possibility may thus be considered that ethanol activates the cholinergic afferents causing a release of acetylcholine in the ventral tegemental area leading to a stimulation of nAChRs and thereby excite the mesocorticolimbic dopamine system.

The importance of testosterone for impulsive-like behavior is unclear. Here we studied the effect of testosterone administration during 6 and 14 days (separate experiments) with one, three and five testosterone-filled silastic capsules implanted subcutaneously on shock-induced behavioral inhibition and on flunitrazepam-induced disinhibition in a modified Vogel's drinking conflict model in rats. Alleviation of shock-induced behavioral inhibition has been suggested to reflect impulsive-like behavior and/or anxiolysis. Treatment with the highest testosterone dose used for 6 (Experiment 1) and 14 (Experiment 3) days increased the number of shocks accepted. Testosterone treatment affected serum levels of testosterone and accessory sex organ weights. Flunitrazepam induced behavioral disinhibition in both testosterone-treated (for 14 days) and sham-treated rats. Moreover, testosterone treatment for 14 days resulted in enhanced GABA-induced 36Cl- uptake into synaptoneurosomes as compared to controls. In conclusion, testosterone produces behavioral disinhibition and may enhance brain GABAA receptor function.

This article represents the proceedings of a symposium at the 2002 ISBRA/RSA meeting in San Francisco. The organizers were Kalervo Kiianmaa and Andrey E. Ryabinin. The chairs were Kalervo Kiianmaa and Jörgen A. Engel. The presentations were (1) The role of opioidergic and dopaminergic networks in ethanol-seeking behavior, by Kalervo Kiianmaa and Petri Hyytiä; (2) Interaction between the dopamine systems in the prefrontal cortex and nucleus accumbens during ethanol self-administration, by Herman H. Samson; (3) Neurochemical and behavioral studies on ethanol and nicotine interactions, by Jörgen A. Engel, Lennart Svensson, Bo Söderpalm, and Anna Larsson; (4) Involvement of the GABA receptor in alcohol reinforcement in sP rats, by Giancarlo Colombo and Giovanni Vacca; (5) Neuroactive steroids and ethanol reinforcement, by Deborah A. Finn, and (6) Potential contribution of the urocortin system to regulation of alcohol self-administration, by Andrey E. Ryabinin and Ryan K. Bachtell.(B)

This study investigated the effects of repeated daily (15 days) treatment with nicotine, alone or in combination with the 5-HT1A/7 receptor agonist (±)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) or the 5-HT2 receptor agonist (±)-2,5-dimethoxy-4-iodoamphetamine (DOI) on locomotor sensitization, mesolimbic dopamine neurochemistry and on behavioral inhibition in the rat. Acute nicotine elevated the extracellular dopamine levels in the nucleus accumbens and stimulated locomotor activity, effects that were sensitized after repeated nicotine treatment. Repeated nicotine administration also produced nicotine-induced behavioral disinhibition in the elevated plus-maze. Treatment with DOI counteracted the expression of the nicotine-induced locomotor and neurochemical sensitization, but had no effect on nicotine-induced behavioral disinhibition. Treatment with 8-OH-DPAT decreased the expression of nicotine-induced behavioral disinhibition, but had no effect on locomotor or neurochemical sensitization. Taken together, these findings suggest that the 5-HT1A and the 5-HT2 receptor subtypes are differentially involved in the effects of repeated nicotine on locomotor sensitization, behavioral inhibition and mesolimbic dopamine neurochemistry.

This study investigated the effects of repeated daily nicotine (0.35 mg/kg; 15 days) treatment on behavioral inhibition and locomotor activity in the elevated plus-maze and on voluntary ethanol consumption. When challenged with nicotine before the test, rats pretreated with repeated nicotine spent more time on and made more entries onto the open arms of an elevated plus-maze than did vehicle-pretreated animals. The ethanol preference and intake, measured during 3 h after a nicotine injection, was also higher in the nicotine-pretreated animals. In ethanol consumption experiments, there was a positive correlation between the % time and % entries made onto open arms vs. the ethanol preference and intake. However, no correlation between the total number of entries made in the elevated plus-maze and the measures of ethanol consumption was observed. These findings suggest that the ability of repeated nicotine administration to increase ethanol consumption is related to development of a nicotine-induced reduction of inhibitory control rather than development of locomotor sensitization.

Impulsive and aggressive behaviors in, e.g., personality or substance abuse disorders in man and corresponding behaviors in rats may involve serotonin (5-HT), γ-amino-butyric acid A /benzodiazepine receptor complexes (GABA A /BDZ-RC) and steroid hormones, e.g., testosterone. Here, we studied the effect of gonadectomy on disinhibitory behavior in individually housed 5-HT-depleted rats and on GABA A /BDZ-RC function in vitro, in corticohippocampal synaptoneurosomes prepared from the brain of these animals. 5-HT depletion by intracerebroventricular 5,7-dihydroxytryptamine (5,7-DHT)-induced disinhibitory behavior in a shock-induced behavioral inhibition model (punished conflict model) 14 days after operation. Gonadectomy in connection with the 5-HT depletion reduced the disinhibitory behavior and testosterone substitution prevented this effect. Shock threshold and drinking motivation were not affected by gonadectomy and/or 5-HT depletion. The relative epididymides weight was increased in 5-HT-depleted as compared to sham-operated rats. However, the serum concentrations of testosterone and the relative testes weights were not different in 5-HT-depleted rats as compared to controls. GABA-induced (30, 100, 300 μM) 36 Cl − -uptake into synaptoneurosomes was lower in 5,7-DHT+gonadectomized rats compared to the control group. This effect was reversed by substitution with testosterone. These results demonstrate that gonadectomy reduces disinhibitory behavior in 5-HT-depleted rats and that GABA A /BDZ-RC may be involved in this effect.

The effect of neonatal brain infection of herpes simplex virus type 1 (HSV-1) on the development of sensorimotor function in the rat was investigated using an acoustic startle paradigm. Intracerebral inoculation of HSV-1 at day 2 after birth, but not at day 4, caused a significant delay in the development of prepulse inhibition of acoustic startle. A decrease in prepulse inhibition was shown at 37, 46 and 58 days of age in these rats compared to control rats. No evidence was obtained for other behavioural dysfunctions such as differences in sensorimotor reactivity, sensorimotor response habituation, spontaneous locomotor activity, rearing activity or stereotyped behaviour. Prepulse inhibition of acoustic startle is an accepted model of sensorimotor gating in the CNS, a function which has been shown diminished in schizophrenic persons. The present results suggest that early viral infections during a neurone-susceptible period may contribute to the development of this deficit.

Prepulse inhibition of acoustic startle is the normal reduction in startle response to an intense auditory stimulus when this stimulus is immediately preceded by a weaker prestimulus. Previous studies have shown that several neuroanatomical structures and pathways in the brain are involved in the modulation of prepulse inhibition. In the present study, the functional importance of the medial geniculate body (MG) in the modulation of prepulse inhibition was investigated. To this end, in vivo brain microdialysis probes were used to infuse drugs locally into the MG of awake, freely moving rats simultaneously with startle response and prepulse inhibition measurements in the same animals. Intrageniculate infusion of the sodium channel blocker, tetrodotoxin, significantly reduced prepulse inhibition without affecting baseline startle amplitude. A similar effect was obtained after intrageniculate infusion of the GABA(B) receptor agonist, baclofen. In addition, intrageniculate infusion of muscimol, an agonist at the GABA(A) receptor complex, reduced prepulse inhibition, although this effect was obtained at a higher concentration of the drug compared to that of baclofen. These studies suggest that the MG is involved in the modulation of prepulse inhibition and that auditory signals relayed via the MG may be subjected to inhibitory control at this level, involving GABA neurotransmission.

Central and peripheral administration of DDAVP increase locomotor activity in rats in doses that alter brain dopamine neurochemistry. In order to delineate the role of catecholamines in this behavioural effect of DDAVP, the effects of different catecholamine manipulating agents on DDAVP-induced locomotor stimulation were studied in rats. The catecholamine depleting agent reserpine (5 mg/kg), administered alone or together with the catecholamine synthesis inhibitor alpha-methyltyrosine (250 mg/kg), completely prevented the locomotor stimulatory effect of DDAVP. The dopamine D1 receptor antagonist Sch-23390 (0.01 and 0.03 mg/kg) significantly antagonized the DDAVP-induced locomotor stimulation when administered in the higher dose, that also produced a significant reduction of locomotor activity per se, whereas the dopamine D2 receptor antagonist raclopride (0.08 and 0.16 mg/kg) had no significant effect. The two dopamine blockers administered together produced a significant, dose-dependent reduction of DDAVP-induced locomotor stimulation, while controls were not significantly affected. Also the alpha-adrenoceptor antagonist phenoxybenzamine decreased the DDAVP-induced locomotor stimulation in a dose (20 mg/kg) that did not influence locomotor activity in controls, and, finally, administration of Sch-23390, raclopride and phenoxybenzamine antagonised the DDAVP-induced effect in a dose combination that failed to influence locomotor activity per se. In vivo microdialysis experiments in awake, freely moving rats indicated that DDAVP increases dopamine overflow in the nucleus accumbens, a brain area of importance for initiation of locomotor activity, by approximately 25%, as compared to baseline levels. Taken together, these results indicate that the central stimulatory action of DDAVP involves granula-mediated dopamine release and subsequent activation of dopamine D1 and D2 receptors, and that alpha-adrenoceptors possibly also are involved.

The mesocorticolimbic dopamine system is believed to be involved in mediating the positive reinforcing effects of drugs of abuse, including ethanol. The nicotinic acetylcholine receptor antagonist mecamylamine perfused via reversed microdialysis in the ventral tegmental area antagonizes the increase of accumbal extracellular dopamine levels after systemic ethanol, and, after systemic injection, lowers ethanol intake in the rat. In the present study the effect of ventral tegmental mecamylamine on ethanol intake and preference, as well as on extracellular accumbal dopamine levels, was investigated in the same animal. To this end, in vivo microdialysis using a double probe approach (one in the nucleus accumbens and one in the ventral tegmental area) was combined with an ethanol preference model invoking a free choice between a bottle of water and a bottle of ethanol 6% (v/v) solution. Wistar rats drinking more than 60% of their total daily fluid intake from the ethanol solution (ethanol high-preferring animals) were selected during a screening period and used for the experiments. The animals received vehicle or mecamylamine (100 microM) in the ventral tegmental area and were then presented with a choice between water and ethanol in a limited access paradigm to which they previously had been adapted. On the next day the rats that received vehicle day 1 now received mecamylamine, and vice versa. When treated with vehicle, ethanol intake and preference were unaltered, as compared to baseline behavior, and accumbal dopamine levels increased significantly to approximately 130% of the pre-drug baseline level. When receiving mecamylamine, ethanol intake and preference were reduced markedly and dopamine levels were unaltered, as compared to pre-drug baseline levels. The present results further indicate that nicotinic acetylcholine receptors in the ventral tegmental area are involved in the positive reinforcing effects of ethanol. Thus, mecamylamine or other antagonists specifically aimed at ventral tegmental nicotinic acetylcholine receptors could represent a new pharmacological treatment principle against alcohol abuse, the efficacy of which should be explored in high-scale alcohol consumers or alcoholics.

Glucagon-like peptide 1 (GLP-1) is an incretine hormone that controls consummatory behavior and glucose homeostasis. It is released in response to nutrient ingestion from the intestine and production in the brain has also been identified. Given that GLP-1 receptors are expressed in reward areas, such as the nucleus accumbens and ventral tegmental area, and that common mechanisms regulate food and drug-induced reward we hypothesize that GLP-1 receptors are involved in reward regulation. Herein the effect of the GLP-1 receptor agonist Exendin-4 (Ex4), on amphetamine- and cocaine-induced activation of the mesolimbic dopamine system was investigated in mice. In a series of experiments we show that treatment with Ex4, at a dose with no effect per se, reduce amphetamine- as well as cocaine-induced locomotor stimulation, accumbal dopamine release as well as conditioned place preference in mice. Collectively these data propose a role for GLP-1 receptors in regulating drug reward. Moreover, the GLP-1 signaling system may be involved in the development of drug dependence since the rewarding effects of addictive drugs involves interferences with the mesolimbic dopamine system. Given that GLP-1 analogues, such as exenatide and liraglutide, are clinically available for treatment of type II diabetes, we propose that these should be elucidated as treatments of drug dependence.

It has been suggested that the mesolimbic dopamine activating and the reinforcing properties of ethanol involve activation of central nicotinic acetylcholine receptors. To test this hypothesis, the effects of two nicotinic receptor antagonists and of subchronic nicotine treatment on voluntary ethanol consumption (ethanol 6% v/v or water) were studied in ethanol low-, medium- or high-preferring Wistar rats. After systemic mecamylamine (2 mg/kg) but not hexamethonium (10 mg/kg) high- but not low-preferring rats decreased their ethanol intake but, however, not their ethanol preference. When subchronically exposed to nicotine (0.35 mg/kg, s.c. daily) medium-preferring rats markedly increased their ethanol intake and preference. This effect lasted for more than 1 week after interrupting nicotine administration. Ethanol intake levels did not correlate with locomotor activity scores after nicotine challenge (0.35 mg/kg, s.c.) or with exploratory locomotor activity. However, exploratory locomotor activity correlated with locomotor activity scores both after nicotine (0.35 mg/kg, s.c.) and ethanol (0.125 g/kg i.p.) challenge. Dopamine release, as indicated by accumulation of 3-methoxytyramine after monoamine oxidase inhibition, was increased in the limbic forebrain (including the nucleus accumbens, the olfactory tubercles, the amygdala and the septum) after acute nicotine (0.35 mg/kg s.c.) or ethanol (2.5 g/kg i.p.) in animals subchronically exposed to nicotine compared to subchronically vehicle-treated controls. The present results further implicate central nicotinic receptors in the molecular events mediating the reinforcing properties of ethanol, and suggest that subchronic nicotine enhances the responsiveness of mesolimbic dopamine neurons both to nicotine and to ethanol. Clinical implications are discussed.

Nocturnal enuresis is characterized by nocturnal urine volumes exceeding bladder capacity and by inability to wake up to the stimulus of a full bladder. Desmopressin (DDAVP) is believed to be efficient in treating nocturnal enuresis by reducing nocturnal urine production. However, clinical observations indicate an additional mode of action since the drug appears to modify sleep architecture, apparently improving the patient's ability to awaken to the stimulus of a full bladder. Because of this, a possible arousing effect of DDAVP was studied.The tentative ability of DDAVP and the endogenous hormone vasopressin (AVP) to produce locomotor stimulation in resting rats after both intracerebroventricular and subcutaneous administration was used as an animal model of arousal. In addition brain monoamine biochemistry was analyzed.The intracerebroventricular injection of AVP (0.1 and 1 microgram.) and the intracerebroventricular (0.1, 1, 10 and 100 microgram.) and subcutaneous (90 and 180 microgram.) injections of DDAVP were both associated with a significant increase in the locomotor activity of the animals compared with controls. The biochemical analysis of cerebral monoamines indicated that DDAVP lowers brain dopamine levels after both types of administration.These results suggest that DDAVP exerts a stimulatory effect in the CNS, which is also observed after peripheral administration. There are also indications for an increase in central dopamine turnover which could explain the registered increase in locomotor activity.

This study determined the relationship between ethanol intake and spontaneous and amphetamine-induced locomotor activity. Locomotion was studied in high-preferring (HP; > 70% of total fluid intake consumed as alcohol) and low-preferring (LP; v v ) ethanol solution for 3 weeks. Following an alcohol-free 3-week period, the animals were tested for spontaneous motor activity for 1 h. One week later, locomotion was recorded in the same activity boxes following a subcutanous injection with d-amphetamine sulfate (1 mg/kg). For determination of plasma levels of corticosterone, blood samples were taken immediately after each of the two tests for locomotor activity. There was no difference between HP and LP rats with regard to spontaneous locomotor activity. Neither were there any differences in plasma levels of corticosterone between the groups. Amphetamine stimulated locomotion in both HP and LP rats, but to a significantly greater extent in HP animals. Both groups had higher blood levels of corticosterone after the amphetamine test than after the drug-free test, but the corticosterone increase was significantly larger in the HP than in the LP rats. These data indicate that the same neural substrate (e.g., the mesocorticolimbic dopamine system) may mediate important aspects of both ethanol drinking and amphetamine responsiveness. Individual differences in the properties of this substrate may account for the finding that ethanol drinking and amphetamine responsiveness covary. A possible explanation for this association may be that prior consumption of ethanol sensitizes the neural substrate responsible for amphetamine-induced hyperactivity. Alternatively, hereditary and/or experiential factors may account for the individual variations in the responsiveness of the imesocorticolimbic DA system to ethanol and amphetamine.

Behavioural sensitization has been implicated in the development of addictive behaviour, and several studies suggest that corticosteroids may be involved in this phenomenon. In the present study, the effects of adrenalectomy and steroid replacement treatments on the behavioural sensitization observed after daily injections of nicotine (0.4 mg/kg s.c.) were investigated in the rat. Adrenalectomy completely prevented sensitization to the locomotor stimulating effect of nicotine after repeated injections but did not influence the acute locomotor activating effect of the drug or an already established sensitization to nicotine. In adrenalectomized animals receiving replacement treatment with corticosterone or dexamethasone, but not aldosterone, repeated administration of nicotine produced behavioural sensitization. Repeated dexamethasone treatment per se failed, however, to sensitize rats to nicotine. Post mortem neurochemical studies showed that repeated administration of nicotine significantly increased homovanillic acid (HVA) levels, as well as the dihydroxyphenylacetic acid (DOPAC)/dopamine quotient, in the limbic forebrain. Adrenalectomy per se significantly increased HVA levels and tended to elevate the DOPAC/dopamine quotient. When repeatedly treated with nicotine, adrenalectomized rats displayed a higher DOPAC/dopamine quotient, but no significant difference in HVA levels, compared to nicotine-treated sham-operated controls. In the striatum and the cortex, no significant effects of nicotine treatment or adrenalectomy were observed on any of the neurochemical measures. The present results suggest that glucocorticoid (type II) receptor activation is required for induction of sensitization to the locomotor stimulatory effect of nicotine, whereas corticosteroids are not required for the expression of the behavioural sensitization once established. Provided that HVA levels and the DOPAC/dopamine quotient relatively well reflect the presynaptic dopamine activating effect of nicotine, it may be suggested that corticosteroid-related mechanisms associated with behavioural sensitization to nicotine are post- rather than presynaptically located in relation to mesolimbic dopamine neurons.

The daily fluid intake of male Wistar rats with simultaneous access to 6% ethanol and water was determined during a baseline period (1 week), following adrenalectomy (1 week) and for 3 weeks following SC implantation of hormone pellets containing corticosterone (CORT) or dexamethasone (DEX). Ethanol consumption dropped during the first week of adrenalectomy (ADX) but increased again in the absence of hormone replacement to reach preoperative levels during the ensuing weeks. The CORT treatment, which produced plasma hormone levels similar to the 24-h mean concentration of adrenally intact rats, not only reversed the effect of ADX on alcohol consumption but also enhanced it to levels above those observed in intact rats. Water intake was not affected by the CORT treatment. DEX implants stimulated water intake, but did not enhance the drinking of ethanol. SC injections of RU 28318 (type I corticosterone receptor antagonist; 10 mg/kg) or mifepristone (RU 38486; type II receptor antagonist; 25 mg/kg) at the beginning and halfway through three daily, 6-h tests failed to affect ethanol drinking in adrenally intact rats or in ADX rats bearing CORT implants. Similarly, there was no effect of giving the two antagonists in combination. These results suggest that exogenous CORT can induce excessive alcohol intake in genetically unselected rats and that this facilitatory effect may be mediated by non-genomic cellular mechanisms.

The affective mimetic responses of male Wistar rats with prior access to 6% ethanol in their home cages were observed during intraoral infusions of an equivalent alcohol solution. Ethanol preference in the home cage appeared unrelated to measures of aversion and ingestion in the taste reactivity tests in normal rats. Adrenalectomy, which significantly reduced home cage ethanol preference, failed to influence the taste reactions elicited by ethanol or water. On the other hand, treatment of intact rats with the 5-HT 1A receptor agonist ipsapirone (2.5 mg/kg), a drug that also decreases ethanol drinking in two-bottle intake tests, did increase the duration of aversive groomings, whereas measures of ingestion remained unaffected. These results suggest that ipsapirone, but not adrenalectomy, may alter the palatibility of ethanol; this perceptual change may partly underlie the ability of ipsapirone to reduce home cage alcohol drinking in the rat.

Several studies report that rats exposed to stressful conditions may increase their ethanol consumption. Stress is accompanied by a rise in the secretion of adrenocortical hormones, and the possibility that these hormones exert an influence on ethanol consumption should be considered. The present investigation addressed this issue by studying the effect of adrenalectomy (ADX) and subsequent corticosterone (CORT) or aldosterone (ALDO) treatment on ethanol intake. The results showed that ADX rats decreased their ethanol intake compared to the sham-operated controls and that treatment with CORT restored the intake of ethanol to the preoperative level. In contrast, treatment with ALDO (0.25 or 0.75 mg/kg) had no effect on ethanol intake. Biochemical analyses showed increases in monoamine turnover in the brain stem and limbic forebrain after ADX. The reduction of ethanol consumption caused by ADX may thus be specifically attributed to the loss of one of the adrenal hormones, CORT. The results indicate that CORT may be a factor of importance in the modulation of alcohol consumption.

Animal studies as well as clinical studies have suggested that the brain 5-HT system is important for the regulation of voluntary ethanol intake and preference. Previous studies have suggested that 5-HT1A receptor agonists may reduce ethanol preference in rats. In the present study on mice, the 5-HT1A receptor agonists (8-OH-DPAT), ipsapirone, and buspirone all antagonized the locomotor activity (LMA) stimulatory effect of ethanol (2.5 g/kg). The present results provide further support for the notion that the LMA-increasing effect of ethanol may be homologous to its reinforcing properties and that 5-HT1A receptor agonists may counteract these properties as well.

The gastric-derived orexigenic peptide ghrelin affects brain circuits involved in energy balance as well as in reward. Indeed, ghrelin activates an important reward circuit involved in natural- as well as drug-induced reward, the cholinergic-dopaminergic reward link. It has been hypothesized that there is a common reward mechanism for alcohol and sweet substances in both animals and humans. Alcohol dependent individuals have higher craving for sweets than do healthy controls and the hedonic response to sweet taste may, at least in part, depend on genetic factors. Rat selectively bred for high sucrose intake have higher alcohol consumption than non-sucrose preferring rats and vice versa. In the present study a group of alcohol-consuming individuals selected from a population cohort was investigated for genetic variants of the ghrelin signalling system in relation to both their alcohol and sucrose consumption. Moreover, the effects of GHS-R1A antagonism on voluntary sucrose- intake and operant self-administration, as well as saccharin intake were investigated in preclinical studies using rodents. The effects of peripheral grelin administration on sucrose intake were also examined. Here we found associations with the ghrelin gene haplotypes and increased sucrose consumption, and a trend for the same association was seen in the high alcohol consumers. The preclinical data show that a GHS-R1A antagonist reduces the intake and self-administration of sucrose in rats as well as saccharin intake in mice. Further, ghrelin increases the intake of sucrose in rats. Collectively, our data provide a clear indication that the GHS-R1A antagonists reduces and ghrelin increases the intake of rewarding substances and hence, the central ghrelin signalling system provides a novel target for the development of drug strategies to treat addictive behaviours.

The mechanisms involved in alcohol use disorders are complex. It has been shown that ghrelin is an important signal for the control of body weight homeostasis, preferably by interacting with hypothalamic circuits, as well as for drug reward by activating the mesolimbic dopamine system. The ghrelin receptor (GHS-R1A) has been shown to be required for alcohol-induced reward. Additionally, ghrelin increases and GHR-R1A antagonists reduce moderate alcohol consumption in mice, and a single nucleotide polymorphism in the GHS-R1A gene has been associated with high alcohol consumption in humans. However, the role of central ghrelin signaling in high alcohol consumption is not known. Therefore, the role of GHS-R1A in operant self-administration of alcohol in rats as well as for high alcohol consumption in Long-Evans rats and in alcohol preferring [Alko alcohol (AA)] rats was studied here. In the present study, the GHS-R1A antagonist, JMV2959, was found to reduce the operant self-administration of alcohol in rats and to decrease high alcohol intake in Long-Evans rats as well as in AA rats. These results suggest that the ghrelin receptor signaling system, specifically GHS-R1A, is required for operant self-administration of alcohol and for high alcohol intake in rats. Therefore, the GHS-R1A may be a therapeutic target for treatment of addictive behaviors, such as alcohol dependence.

International audience; Here we review recent advances that identify a role for the central ghrelin signalling system in reward from both natural rewards (such as food) and artificial rewards (that include alcohol and drugs of abuse). Whereas ghrelin emerged as a stomach-derived hormone involved in energy balance, hunger and meal initiation via hypothalamic circuits, it now seems clear that it also has a role in motivated reward-driven behaviours via activation of the so-called "cholinergic-dopaminergic reward link". This reward link comprises a dopamine projection from the ventral tegmental area (VTA) to the nucleus accumbens together with a cholinergic input, arising primarily from the laterodorsal tegmental area. Ghrelin administration into the VTA or LDTg activates the "cholinergic-dopaminergic" reward link, suggesting that ghrelin may increase the incentive value of motivated behaviours such as reward-seeking behaviour ("wanting" or "incentive motivation"). Further, direct injection of ghrelin into the brain ventricles or into the VTA increases the consumption of rewarding foods as well as alcohol in mice and rats. Studies in rodents show beneficial effects of ghrelin receptor (GHS-R1A) antagonists to suppress the intake of palatable food, to reduce preference for caloric foods, to suppress food reward and motivated behaviour for food. They have also been shown to reduce alcohol consumption, suppress reward induced by alcohol, cocaine and amphetamine. Further, variations in the GHS-R1A and pro-ghrelin genes have been associated with high alcohol consumption, smoking and increased weight in alcohol dependent individuals as well as with bulimia nervosa and obesity. Thus, the central ghrelin signalling system interfaces neurobiological circuits involved in reward from food as well as chemical drugs; agents that directly or indirectly suppress this system emerge as potential candidate drugs for suppressing problematic over-eating that leads to obesity as well as for the treatment of substance use disorder.

The mechanisms involved in alcohol use disorder, a chronic relapsing brain disorder, are complex and involve various signalling systems in the brain. Recently, the orexigenic peptide ghrelin was shown to be required for alcohol-induced reward, an effect mediated via ghrelin receptors, GHS-R1A, at the level of the cholinergic-dopaminergic reward link. Moreover, ghrelin increases and GHR-R1A antagonists reduce moderate alcohol consumption in mice, and a single nucleotide polymorphism in the GHS-R1A gene has been associated with high alcohol consumption in humans. Therefore, GHS-R1A gene expression and alcohol intake were investigated in high, AA (Alko, Alcohol), versus low, ANA (Alko, Non-Alcohol), alcohol consuming rats as well as in Wistar rats. In the AA and ANA rats plasma ghrelin levels were also measured. GHS-R1A gene expression was increased in AA compared to ANA rats in nucleus accumbens, ventral tegmental area, amygdala, prefrontal cortex and hippocampus. A similar trend was observed in the ventral tegmental area of Wistar rats consuming high amounts of alcohol. Furthermore, the AA rats had significantly smaller reduction of plasma ghrelin levels over time, after several weeks of alcohol exposure, than had the ANA rats. The present study provides further evidence for that the ghrelin signalling system, in particular at the level of the mesocortocolimbic dopamine system, is involved in alcohol consumption, and thus possibly contributes to alcohol use disorder. Therefore the GHS-R1A may constitute a novel candidate for development of new treatment strategies for alcohol dependence.

The neuropeptide galanin is present in limbic brain areas important for emotionality. We examined whether central galanin may be involved in mechanisms of anxiety. Rats were tested in a pharmacologically validated animal model of anxiety, the Vogel punished drinking test. A 100% increase of punished responding was seen after 3 nmol galanin i.c.v. After 15 nmol, signs of sedation were seen, and no increase of punished responding could be observed. Drinking motivation, shock thresholds and exploratory locomotor activity were not affected by 3 nmol galanin. These results support a specific anxiolytic-like action of galanin, similar to that of the functionally related peptide, neuropeptide Y.

A two-bottle, free-choice paradigm was used to investigate the influence of the serotonergic (5-HT) system on ethanol intake in genetically heterogeneous Wistar rats. Systemic administration of the 5-HT 1A agonist ipsapirone (1.25–5.0 mg/kg) caused a dose-dependent decrease in ethanol preference and intake, while the 5-HT 2 antagonist ritanserin (1.25–5.0 mg/kg) and the 5-HT 3 antagonists ondansetron (0.01–1.0 mg/kg) and granisetron (0.5–1.0 mg/kg) failed to alter ethanol consumption. The effect of ipsapirone treatment on ethanol intake was more pronounced in high-preferring animals than in low-preferring. A closer look at the microstructure of the rat's drinking behaviour by means of a microcomputer-controlled data acquisition system showed that ipsapirone treatment caused a significant decrease in the number of licks recorded at the ethanol-containing bottle and a decrease in the time spent at this bottle. Furthermore, ipsapirone treatment caused a significant increase in the number of breaks in licking behaviour recorded at this bottle. The drinking behaviour at the water-containing bottle was not affected by the ipsapirone treatment. Neither was the rat's eating behaviour altered by this treatment. These findings support the hypothesis that the 5-HT system is involved in the regulation of ethanol intake, with special emphasis on the involvement of the 5-HT 1A receptor subtype, and may indicate that central reward-mediating mechanisms are influenced.

Recently, we demonstrated that the central ghrelin signalling system, involving the ghrelin receptor (GHS-R1A), is important for alcohol reinforcement. Ghrelin targets a key mesolimbic circuit involved in natural as well as drug-induced reinforcement, that includes a dopamine projection from the ventral tegmental area (VTA) to the nucleus accumbens. The aim of the present study was to determine whether it is possible to suppress ghrelin's effects on this mesolimbic dopaminergic pathway can be suppressed, by interrupting afferent inputs to the VTA dopaminergic cells, as shown previously for cholinergic afferents. Thus, the effects of pharmacological suppression of glutamatergic, orexin A and opioid neurotransmitter systems on ghrelin-induced activation of the mesolimbic dopamine system were investigated. We found in the present study that ghrelin-induced locomotor stimulation was attenuated by VTA administration of the N-methyl-D-aspartic acid receptor antagonist (AP5) but not by VTA administration of an orexin A receptor antagonist (SB334867) or by peripheral administration of an opioid receptor antagonist (naltrexone). Intra-VTA administration of AP5 also suppressed the ghrelin-induced dopamine release in the nucleus accumbens. Finally the effects of peripheral ghrelin on locomotor stimulation and accumbal dopamine release were blocked by intra-VTA administration of a GHS-R1A antagonist (BIM28163), indicating that GHS-R1A signalling within the VTA is required for the ghrelin-induced activation of the mesolimbic dopamine system. Given the clinical knowledge that hyperghrelinemia is associated with addictive behaviours (such as compulsive overeating and alcohol use disorder) our finding highlights a potential therapeutic strategy involving glutamatergic control of ghrelin action at the level of the mesolimbic dopamine system.

We investigated whether ghrelin action at the level of the ventral tegmental area (VTA), a key node in the mesolimbic reward system, is important for the rewarding and motivational aspects of the consumption of rewarding/palatable food. Mice with a disrupted gene encoding the ghrelin receptor (GHS-R1A) and rats treated peripherally with a GHS-R1A antagonist both show suppressed intake of rewarding food in a free choice (chow/rewarding food) paradigm. Moreover, accumbal dopamine release induced by rewarding food was absent in GHS-R1A knockout mice. Acute bilateral intra-VTA administration of ghrelin increased 1-hour consumption of rewarding food but not standard chow. In comparison with sham rats, VTA-lesioned rats had normal intracerebroventricular ghrelin-induced chow intake, although both intake of and time spent exploring rewarding food was decreased. Finally, the ability of rewarding food to condition a place preference was suppressed by the GHS-R1A antagonist in rats. Our data support the hypothesis that central ghrelin signaling at the level of the VTA is important for the incentive value of rewarding food.

An in vivo voltammetry technique was used to monitor the extracellular ascorbate (AA) concentration in the nucleus accumbens and striatum of unanesthetized, freely moving rats. A single injection of ethanol, 1.0 g/kg intraperitoneally (IP), induced a significant increase in extracellular AA concentration in both the nucleus accumbens and striatum. This effect was dose dependent within a dose range from 0.5–2.0 g/kg. 4-Methylpyrazole (50 mg/kg, IP), which inhibits alcoholdehydrogenase, could not prevent the increase in AA concentration, evoked by ethanol. Furthermore, systemic administration of acetaldehyde (20 mg/kg, IP), the main metabolite of ethanol, did not have any effect on the level of AA in the nucleus accumbens or striatum. These results show that ethanol can alter the brain extracellular AA levels and that this effect seems to be attributed to ethanol itself and not to acetaldehyde. Consequently, these results indicate that a role for AA in the action of ethanol in the brain should be considered.

In vivo voltammetry at electrochemically pretreated carbon fibre electrodes was used to investigate the effect of intracerebroventricular (i.c.v.) administration of N-methyl-D-aspartic acid (NMDA) on neuronal activity in rat nucleus accumbens. Infusion of a low dose of NMDA (1 nmol) was followed a few minutes later by rapid changes in both Peak 1 and Peak 2 heights indicating large but short-lived increases in the extracellular concentrations of ascorbate and catecholamines, respectively. These responses did not seem to be dependent on the dose infused since infusion of NMDA for a longer time period neither changed the amplitude nor the time-course of these effects. The increase in Peak 2 height was resistant to pargyline pretreatment indicating that this response mainly reflected the release of dopamine. The administration of NMDA was followed by behavioural activation in the animals but not convulsions. Co-administration of the competitive NMDA receptor antagonist, CPP (1 nmol), completely blocked these effects while the acetylcholine receptor antagonist, atropine (1.5 nmol), and the GABA receptor antagonist, picrotoxin (1 nmol), failed in this respect. The phenomenon spreading depression is discussed as a possible explanation of these results.