Blockade of the Corticotropin Releasing Factor Type 1 Receptor Attenuates Elevated Ethanol Drinking Associated With Drinking in the Dark Procedures

Rodent models of alcoholism, including inbred and selectively bred strains have been useful tools for identifying the genetic and neurobiological factors that underlie this disease. However, in many cases, rodents do not consume enough alcohol to reach the point of behavioral and/or pharmacological intoxication (Spanagel, 2000). Recently, “drinking in the dark” (DID) procedures have been developed to induce excessive ethanol drinking in C57BL/6J mice, which result in blood ethanol concentrations (BECs) reaching levels that have measurable effects on physiology and/or behavior (Rhodes et al., 2005, 2007). With these procedures, C57BL/6J mice are given access to a 20% ethanol solution for 2 to 4 hours, starting with 3 hours into their dark cycle. C57BL/6J can achieve BECs of >100 mg% and exhibit signs of behavioral intoxication as measured by motor deficits on the rotarod and balance beam (Rhodes et al., 2005, 2007). It has been argued that the DID model has predictive validity for testing potential pharmacological targets aimed at treating alcohol abuse disorders as naltrexone, an opioid receptor antagonist which is currently used to treat alcoholism, dose dependently attenuates high levels of ethanol drinking induced by DID procedures (Kamdar et al., 2007). Corticotropin releasing factor (CRF) is a 41 amino acid neuromodulator that is widely expressed throughout the central nervous system (Bloom et al., 1982; Merchenthaler et al., 1982). CRF has been shown to modulate diverse biological functions including food intake, stress and anxiety-like behaviors, and neurobiological responses to ethanol [for reviews, see (Heilig and Koob, 2007; Valdez, 2006; Zorrilla and Koob, 2004; Zorrilla et al., 2003)]. Increases in CRF immunoreactivity (Olive et al., 2002; Zorrilla et al., 2001) and levels of extracellular CRF (Funk et al., 2006) are seen in the amygdala following ethanol withdrawal. Exposure to ethanol causes robust activation of the hypothalamic-pituitary-adrenal (HPA)-axis (Rivier, 1996; Rivier et al., 1990), which is initiated by ethanol-induced increases of CRF activity within the hypothalamus (Li et al., 2005; Rivier and Lee, 1996). Recent pharmacological and genetic evidence support the hypothesis that CRF exerts its effects on ethanol consumption through activation of the CRF1 receptor (CRF1R). Blockade of the CRF1R attenuates ethanol intake in dependent, but not nondependent rodents (Funk et al., 2007; Gehlert et al., 2007). Consistently, CRF1R deficient mice failed to show increased ethanol consumption following the acquisition of ethanol dependence and a period of abstinence that was observed in wild-type mice (Chu et al., 2007). Interestingly, a genetic polymorphism at the Crhr1 locus, which encodes the CRF1R was found to be significantly linked to alcoholism (Treutlein et al., 2006). Because CRF receptor signaling has been implicated in a wide range of neurobiological responses to ethanol, the goal of the present set of experiments was to determine whether the increased consumption of ethanol associated with DID procedures can be modulated by pretreatment with CP-154,526, a CRF1R antagonist. Specifically, because ethanol triggers HPA-axis signaling which is initiated by ethanol-induced increases of CRF activity within the hypothalamus (Li et al., 2005; Rivier and Lee, 1996), and because levels of corticosterone, a HPA-axis-associated hormone, have been shown to positively correlated with ethanol intake (Fahlke et al., 1994, 1995, 1996), we predicted that CRF1R blockade would attenuate increased ethanol drinking promoted by DID procedures.