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    • Beside these studies related to food several groups examined

    2022-07-04

    Beside these studies related to food, several groups examined the role of ghrelin in the actions of psychostimulants. Modulation of the ghrelin system and food restriction alter locomotor sensitization, DA release in the NAc, and CPP induced by cocaine, amphetamine, and nicotine (Abizaid et al., 2011, Clifford et al., 2011, Clifford et al., 2012, Davis et al., 2007, Jerlhag et al., 2010, Jerlhag and Engel, 2011, Wellman et al., 2005, Wellman et al., 2011, Zheng et al., 2013). Moreover, GHSR1a atp enzyme diminishes the reinforcement for intracranial self-stimulation with an electrode implanted in the lateral hypothalamic area (LHA, Wellman et al., 2012).
    Ghrelin and the mesolimbic dopamine pathway Many studies indicate that DA is the main mediator of the actions of ghrelin on the reward system. DA is implicated in reward-associated behaviors and the mesolimbic pathway is a key component of food motivational aspects (Palmiter, 2007). Ghrelin activates the mesolimbic DA pathway (Abizaid et al., 2006, Jerlhag et al., 2006, Jerlhag et al., 2007). Ghrelin administration into the VTA induces feeding (Abizaid et al., 2006, Jerlhag et al., 2006, Naleid et al., 2005, Skibicka et al., 2011) and enhances palatable food intake (Egecioglu et al., 2010, Perello et al., 2010). The orexigenic action of ghrelin or food deprivation is blunted by the administration of a ghrelin antagonist in the VTA (Abizaid et al., 2006, Denis et al., 2015). Furthermore, ghrelin administered in the VTA increases the effort that rats make to obtain a food reward (Skibicka et al., 2011). Highly palatable food-related behaviors are impaired in VTA-lesioned rats or after ghrelin receptors blockade in the VTA, but not in the NAc, suggesting the role of ghrelin action in the VTA (Abizaid et al., 2006, Denis et al., 2015, Egecioglu et al., 2010, Skibicka et al., 2011). Interestingly, the positive effects of ghrelin on food intake are seen only when one type of food is presented. In contrast, ghrelin can change the preference from high fat food to chow in a binge eating protocol or a free choice paradigm, suggesting its implication in food choice (Bake et al., 2017, Schele et al., 2016). This change in food preference was reproduced by an injection of ghrelin in the VTA (Bake et al., 2017). These results suggest a specific role of the VTA for ghrelin actions on both food motivation and choice. Ghrelin potentiates DA release in the NAc induced by food or food predictive cues, an effect that seems to depend on orexin in the VTA (Cone et al., 2014, Cone et al., 2015). Food cues increase VTA DA neurons firing in food-restricted rats but ghrelin injected i.p. does not modify this effect (van der Plasse et al., 2015). In support of the role of VTA to NAc DA pathway, the injection of an antagonist of either D1 or D2 DA receptor in the NAc blocked the actions of ghrelin in the VTA on food reward behavior but not chow intake (Skibicka et al., 2013). Ghrelin administration into the VTA increases DA neurons activity (Abizaid et al., 2006, Jerlhag et al., 2006, Jerlhag et al., 2007). Ghrelin action on DA neurons is mediated, at least in part, by cholinergic neurons since ghrelin increases acetylcholine release in the VTA, thereby activating DA neurons and inducing DA release in the NAc (Jerlhag et al., 2012, Fig. 2). This suggested acetylcholine-DA link could also be involved in ghrelin actions on locomotor activity (Jerlhag et al., 2006). In addition, ghrelin may act directly on DA neurons since GHSR1a is expressed in about half of them in the VTA (Abizaid et al., 2006, Guan et al., 1997, Zigman et al., 2006). Although most actions of ghrelin on DA neurons seem to originate in the VTA, GHSR1a also appears to co-localize with DA D1 receptors in several brain areas where they may form heterodimers (Jiang et al., 2006). GHSR1a were also proposed to heteromerize with DA D2 receptors in hypothalamic neurons and thereby regulate appetite in vivo (Kern et al., 2012). Few studies, however, addressed the possible direct effects of ghrelin in the NAc. Ghrelin injected into the NAc moderately increases food intake (Naleid et al., 2005) and potentiates cocaine-induced locomotor activity (Jang et al., 2013).