Abstract
In murine models of alcoholism, the glutamate receptor scaffolding protein Homer2 bidirectionally regulates alcohol intake. Although chronic alcohol drinking increases Homer2 expression within the core subregion of the nucleus accumbens (NAc) of alcohol-preferring P rats, the relevance of this neuroadaptation for alcohol intake has yet to be determined in rats. Thus, the present study employed an adeno-associated viral vector (AAV) strategy to over-express and knock down the major rodent isoform Homer2b within the NAc of both P and outbred Wistar rats to examine for changes in alcohol preference and intake (0–30% v/v) under continuous-access procedures. The generalization of AAV effects to non-drug, palatable, sweet solutions was also determined in tests of sucrose (0–5% w/v) and saccharin (0–0.125% w/v) intake/preference. No net-flux in vivo microdialysis was conducted for glutamate in the NAc to relate Homer2-dependent changes in alcohol intake to extracellular levels of glutamate. Line differences were noted for sweet solution preference and intake, but these variables were not affected by intra-NAc AAV infusion in either line. In contrast, Homer2b over-expression elevated, while Homer2b knock-down reduced, alcohol intake in both lines, and this effect was greatest at the highest concentration. Strikingly, in P rats there was a direct association between changes in Homer2b expression and NAc extracellular glutamate levels, but this effect was not seen in Wistar rats. These data indicate that NAc Homer2b expression actively regulates alcohol consumption by rats, paralleling this previous observation in mice. Overall, these findings underscore the importance of mesocorticolimbic glutamate activity in alcohol abuse/dependence and suggest that Homer2b and/or its constituents may serve as molecular targets for the treatment of these disorders.
Highlights
The Homer family of post-synaptic scaffolding proteins are encoded by 3 genes (Homer1, 2, 3), which give rise to both constitutively expressed and activity-induced genes products (c.f., Shiraishi-Yamaguchi & Furuichi, 2007)
Sections from associated viral vector (AAV)-cDNA infused rats were stained with an antibody against the hemagglutinin (HA) tag using a mouse anti-HA primary antibody (1:1000 dilution; Covance, San Diego, CA) using standard immunohistochemical techniques and examined for vector spread under a light microscope, while sections from AAV-GFP and AAV-shRNA infused rats were examined for GFP immunofluorescence under a fluorescent microscope (e.g., Cozzoli et al, 2009; 2012, 2014; Gould et al, 2015; Goulding et al, 2011; Szumlinski et al, 2004; 2006)
Immunoblotting conducted on the tissue surrounding the microinjector tips within the NAcC revealed changes in Homer2a/b expression appropriate for the infused construct with AAV-cDNA and AAV
Summary
The Homer family of post-synaptic scaffolding proteins are encoded by 3 genes (Homer1, 2, 3), which give rise to both constitutively expressed and activity-induced genes products (c.f., Shiraishi-Yamaguchi & Furuichi, 2007). The capacity to multimerize is critical for the ability of Homer proteins to regulate the functional architecture of excitatory synapses To this end, constitutively expressed Homer proteins play active roles in regulating dendritic morphology, intracellular signaling through both Group mGluRs and NMDA receptors, the integration of intracellular calcium signals, extracellular glutamate, and stimulated glutamate release, as well as the generation of anadamide (e.g., Iasevoli, Tomasetti, & de Bartolomeis, 2013; Jung et al, 2007; Shiraishi-Yamaguchi & Furuichi, 2007; Szumlinski, Ary, & Lominac, 2008).
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