GDNF is an Endogenous Negative Regulator of Ethanol-Mediated Reward and of Ethanol Consumption After a Period of Abstinence

GDNF is a secreted protein that was initially identified in a glial-derived cell line (Lin et al., 1993) and has since been shown to be highly expressed in neurons (Barroso-Chinea et al., 2005; Pochon et al., 1997). GDNF is expressed throughout the central nervous system during development (Choi-Lundberg and Bohn, 1995; Nosrat et al., 1996; Schaar et al., 1993; Stromberg et al., 1993), and high levels of GDNF mRNA are still present in adult spinal cord and specific brain areas, including the striatum (dorsal striatum and nucleus accumbens), thalamus, cortex and hippocampus (Barroso-Chinea et al., 2005; Choi-Lundberg and Bohn, 1995; Golden et al., 1998, 1999; Nosrat et al., 1996; Pochon et al., 1997; Springer et al., 1994; Trupp et al., 1997). GDNF plays an important role in promoting the survival, regeneration and maintenance of the mature phenotype of distinct central and peripheral neuronal populations, such as midbrain dopaminergic neurons (Beck et al., 1995; Granholm et al., 2000; Kowsky et al., 2007; Lin et al., 1993; Pascual et al., 2008; Tomac et al., 1995a). GDNF also promotes neurite outgrowth (Kowsky et al., 2007; Toledo-Aral et al., 2003), and contributes to some cognitive functions (Gerlai et al., 2001). GDNF acts through a multicomponent receptor system, which includes the GDNF family receptor alpha-1 (GFRα1) and the tyrosine kinase receptor Ret. Formation of the GDNF-GFRα1 signaling complex leads to the autophosphorylation of Ret and the subsequent activation of downstream signaling cascades, such as the mitogen-activated protein kinase (MAPK), phosphoinositide 3-kinase and phospholipase Cγ/protein kinase C pathway (Airaksinen and Saarma, 2002; Sariola and Saarma, 2003). GFRα1 and Ret are expressed in several brain regions in the developing and adult brain and are particularly abundant in the mesencephalic dopaminergic areas projecting to the striatum, namely the substantia nigra pars compacta and the ventral tegmental area (VTA) (Burazin and Gundlach, 1999; Glazner et al., 1998; Golden et al., 1998, 1999; Matsuo et al., 2000; Trupp et al., 1997). Interestingly, GDNF is retrogradely transported to dopamine neurons of the mesencephalic system in the adult brain (Ai et al., 2003; Barroso-Chinea et al., 2005; Kordower et al., 2000; Lapchak et al., 1997; Tomac et al., 1995b), supporting an important role of the endogenous growth factor in the maintenance and functionality of these neurons. In recent years, a role for GDNF as a negative regulator of biochemical and behavioral adaptations to drugs of abuse has been reported (Carnicella and Ron, 2009c). Specifically, Messer et al. showed that infusion of GDNF into the VTA reversed biochemical adaptations to long-term exposure to cocaine and morphine, and blocked cocaine-induced CPP (Messer et al., 2000). Transplantation of cells engineered to over-express GDNF or delivery of nanoparticles conjugated to GDNF into the striatum was shown to reduce acquisition of rat cocaine self-administration (Green-Sadan et al., 2003, 2005), and increasing GDNF expression in the brain reduced the sensitivity of mice to methamphetamine (Niwa et al., 2007b). We found that systemic administration of the naturally occurring alkaloid, Ibogaine, and the ergotamine derivative, FDA approved drug, cabergoline, to mice and rats increases the expression of GDNF in the VTA (Carnicella et al., 2009a; He et al., 2005). We further showed that treatment of dopaminergic-like SH-SY5Y cells with Ibogaine or cabergoline results in the activation of the Ret/MAPK cascade via newly synthesized GDNF (Carnicella et al., 2009a; He et al., 2005). Importantly, we found that Ibogaine and cabergoline, via up-regulation of the GDNF pathway in the VTA, reduces rat operant self-administration of ethanol (Carnicella et al., 2009a; He et al., 2005). We also presented data to suggest that the long-lasting reduction of ethanol self-administration by Ibogaine may be mediated by the activation of an autoregulatory feedback loop in which GDNF up-regulates its own expression (He and Ron, 2006). Furthermore, we reported that direct infusion of GDNF into the VTA of rats results in a specific, rapid and sustained reduction of ethanol self-administration (Carnicella and Ron, 2009c; Carnicella et al., 2008, 2009b). We further showed that the actions of GDNF to attenuate the motivation to consume ethanol are mediated via the activation of the MAPK pathway (Carnicella et al., 2008). Finally, we found that GDNF inhibits the chronic ethanol-mediated increase in tyrosine hydroxylase immunoreactivity in the SH-SY5Y cells (He and Ron, 2008), suggesting that GDNF may also act to reduce cellular biochemical adaptations upon prolonged exposure to ethanol. In line with these data, various studies have suggested a role for endogenous GDNF in mechanisms that underlie sensitivity of rodents to drugs of abuse. This possibility stems from studies using GDNF heterozygote knockout mice (GDNF HET) (Granholm et al., 1997; Pichel et al., 1996), as homozygote knockout mice die at birth from renal abnormalities (Moore et al., 1996; Pichel et al., 1996; Sanchez et al., 1996). GDNF HET mice have lower (35 to 70%) GDNF protein levels in the striatum compared with wild-type (WT) mice (Airavaara et al., 2004; Griffin et al., 2006; Niwa et al., 2007b; Yan et al., 2007), and show greater sensitivity to some effects of drugs of abuse. For example, GDNF HET mice exhibit greater place preference to morphine (Niwa et al., 2007a; but see Airavaara et al., 2007), cocaine (Messer et al., 2000), and methamphetamine (Niwa et al., 2007b), and are more vulnerable to psychomotor sensitization induced by morphine (Airavaara et al., 2007) and cocaine (Messer et al., 2000; but see Airavaara et al., 2004). In addition, administration of morphine, but not cocaine, induces a greater increase in striatal dopamine levels in GDNF HET mice than in WT mice (Airavaara et al., 2004, 2006, 2007). Furthermore, motivation to self-administer and to seek methamphetamine is potentiated in GDNF HET mice (Yan et al., 2007). Together, these studies suggest an important role for the endogenous GDNF system in the regulation of behavioral phenotypes associated with exposure to drugs of abuse. In this study, we set out to test for the interaction between endogenous GDNF and ethanol’s actions by determining the sensitivity of GDNF and GFRα1 HET mice and their WT littermates to ethanol.