The galanin-3 receptor antagonist, SNAP 37889, reduces operant responding for ethanol in alcohol-preferring rats

Abstract

Background/objective

The galanin-3 receptor (GALR3) subtype has been identified as having a role in both feeding behaviour and the regulation of emotional states including anxiety. Despite the evidence for an association between galanin and alcohol, the current study is the first to explore the direct role of GALR3 in this context. The present study investigated the potential of the novel selective GALR3 antagonist, SNAP 37889, to reduce anxiety-like behaviour and voluntary ethanol consumption in the iP (alcohol-preferring) rat. This was achieved through a number of behavioural paradigms testing for anxiety, along with the operant self-administration model.

Results

Overall, male iP rats treated with SNAP 37889 at a dose of 30 mg/kg (i.p.) did not show altered locomotor activity or changes in anxiety-like behaviour in the elevated plus maze or light–dark paradigms. Treatment with SNAP 37889 (30 mg/kg, i.p.) reduced operant responding for solutions containing ethanol, sucrose and saccharin. Collectively, results from the current study showed that SNAP 37889 (30 mg/kg, i.p.) is effective in reducing operant responding for ethanol, independent of a sedative effect.

Conclusions

These findings provide evidence that GALR3 antagonism reduces alcohol consumption and further suggest that GALR3 may be implicated in the rewarding effects of natural and drug reinforcers.

Introduction

Alcohol is the most widely used recreational drug in Australia, with an average of 7.2 l consumed annually per person [1]. In 2008, an alarming 8.6% of Australians self-reported that they drank at levels considered to be of risk to their health [2]. In 1998–1999, the cost of alcohol-related social problems in Australia was estimated at $7.6 billion [3], but this figure does not take into account the incalculable and devastating human cost.

The social and economic burden of alcohol abuse has prompted much research into understanding the underlying mechanisms of alcohol and how it interacts with the brain to ultimately cause addictive behaviour and dependence. Recently, the neuropeptide galanin has been identified as a possible target for new therapeutic agents to treat alcoholism [4], [5], [6] and also as a possible therapeutic target for the development of antidepressant and anxiolytic drugs [7], [8].

Galanin is synthesised in brain regions including the dorsal raphe nucleus (DRN), locus coeruleus (LC) [9], rostral ventrolateral medulla (RVLM), central nucleus of the amygdala (CeA), hypothalamic nuclei including the paraventricular nucleus (PVN) and the supraoptic nucleus (SON) [10]. Galanin is also located in a wide range of tissues other than the brain including the spinal cord and gut [11]. The physiological effects of galanin occur through binding to one or more of three identified G-protein coupled receptor (GPCR) subtypes [11]. The unique distribution of galanin receptor subtypes (GALR1, GALR2 and GALR3) through the central nervous system and periphery, allows galanin to mediate a variety of physiological actions, with some crossover due to localisation of receptor subtypes within the same region [7]. With regard to centrally-controlled functions, galanin has been shown to have a variety of effects on feeding, motivation, reward, memory and potentially addiction [7], [12].

GALR3 are distributed most prominently in regions of the rat brain including the hypothalamus, DRN, LC [13] and the amygdala (AMG) [13], [14], which suggests that the major physiological effects resulting from GALR3 activation would be on emotion, mood regulation and feeding. Experimental evidence already indicates that GALR3 is an effective target for reducing depressive and anxiety-like symptoms in a range of animal models as indicated by an attenuation of stress-induced hypothermia in mice, enhanced rat social interaction, an increase in swimming time in rats during the forced swim test [15], [16] and a decrease in time spent immobile in the forced swim test [15]. Previous research has also shown a clear link between galanin and alcohol consumption as injection of galanin into the paraventricular nucleus (PVN) [6] and third ventricle of the brain [4], [6], [17] caused an increase in ethanol intake. Unlike other drugs of abuse, alcohol has a rich caloric content and is therefore postulated to interact with regions of the brain involved in the control of appetite and feeding [18].

Previous research has also shown that galanin knockout mice show a significant decrease in voluntary ethanol intake and preference when compared with wild-type mice [19]. In contrast, galanin over-expressing mice have been found to have an increased intake and preference for ethanol when compared with wild-types, suggesting that galanin plays a role in brain regions that are involved in stimulating ethanol consumption [20]. A previous study also suggested the rewarding effects of alcohol may be mediated by GALR3 located in the hypothalamus, which regulates appetite and feeding [21]. In addition, the same study revealed a link between a single nucleotide polymorphism in the GALR3 gene and an increased risk of alcoholism [21]. There has been no link made between alcoholism and GALR1 or GALR2, which suggests GALR3 may play a more critical role in mediating the functions of galanin related to alcohol use.

Given that the density of GALR3 is highest in regions of the brain that mediate motivation, emotion and feeding [11], targeting these receptors would be more likely to have a positive outcome on alcohol consumption and anxiety when compared with GALR1 and GALR2 receptors. GALR3 has been comparatively understudied in comparison to the other subtypes of receptors due to both the sparse distribution of GALR3 in the brain (in comparison to GALR1 and GALR2 receptors) and the lack of GALR3-selective compounds with high bioavailability [7].

Selective GALR3 antagonists have only recently been synthesised for use in research and while they have been used to investigate the effect of GALR3 blockade on anxiety and depressive-like symptoms, to date no research has investigated the effect of blocking GALR3 in the brain on the consumption of alcohol.

This study therefore aimed to investigate whether the GALR3 antagonist, SNAP 37889 [16], has a positive effect on reducing alcohol consumption and altering anxiety-related behaviour. Potential reductions in alcohol consumption and anxiety were examined through animal models using the iP rat, which has an anxious phenotype and has been shown in many previous studies to voluntarily consume large quantities of alcohol in preference to water [22], [23], [24].