Role of cannabinoid CB 2 receptor in the reinforcing actions of ethanol Antonio Ortega-Álvaro 1 , Alexander Ternianov 1 , Auxiliadora Aracil-Fernández 2,3 , Francisco Navarrete 2,3 , Maria Salud García-Gutiérrez 2,3 & Jorge Manzanares 2,3 Unidad de Neuropsicofarmacología Traslacional, Complejo Hospitalario Universitario de Albacete, Albacete, Spain 1 , Instituto de Neurociencias, Universidad Miguel Hernández-CSIC, San Juan de Alicante, Alicante, Spain 2 and RedTemática de Investigación Cooperativa en Salud (RETICS-Trastornos Adictivos), Instituto de Salud Carlos III, MICINN and FEDER, Madrid, Spain 3 ABSTRACT This study examines the role of the cannabinoid CB2 receptor (CB2r) on the vulnerability to ethanol consumption. The time-related and dose-response effects of ethanol on rectal temperature, handling-induced convulsions (HIC) and blood ethanol concentrations were evaluated in CB2KO and wild-type (WT) mice. The reinforcing properties of ethanol were evaluated in conditioned place preference (CPP), preference and voluntary ethanol consumption and oral ethanol self-administration. Water-maintained behavior schedule was performed to evaluate the degree of moti- vation induced by a natural stimulus. Preference for non-alcohol tastants assay was performed to evaluate the dif- ferences in taste sensitivity.Tyrosine hydroxylase (TH) and μ-opioid receptor gene expressions were also measured in the ventral tegmental area and nucleus accumbens (NAcc), respectively. CB2KO mice presented increased HIC score, ethanol-CPP, voluntary ethanol consumption and preference, acquisition of ethanol self-administration, and increased motivation to drink ethanol compared with WT mice. No differences were found between genotypes in the water-maintained behavior schedule or preference for non-alcohol tastants. Naïve CB2KO mice presented increased μ-opioid receptor gene expression in NAcc. Acute ethanol administration (1–2 g/kg) increased TH and μ-opioid recep- tor gene expressions in CB2KO mice, whereas the lower dose of ethanol decreased TH gene expression in WT mice. These results suggest that deletion of the CB2r gene increased preference for and vulnerability to ethanol consump- tion, at least in part, by increased ethanol-induced sensitivity of the TH and μ-opioid receptor gene expressions in mesolimbic neurons. Future studies will determine the role of CB2r as a target for the treatment of problems related with alcohol consumption. Keywords cannabinoid CB2 receptor, ethanol consumption, ethanol self-administration, knockout mice, μ-opioid receptor, tyrosine hydroxylase. Correspondence to: Jorge Manzanares, Instituto de Neurociencias, Universidad Miguel Hernández-CSIC, Avda. Ramón y Cajal s/n, 03550 San Juan de Alicante, Spain. E-mail: jmanzanares@umh.es INTRODUCTION In recent years, a number of studies have suggested a close interaction between the endocannabinoid system and ethanol-related behaviors (Ortiz et al. 2004a,b; Ishiguro et al. 2007; Oliva & Manzanares 2007; Oliva et al. 2008; Vinod et al. 2008; Linsenbardt & Boehm 2009; Femenía, García-Gutiérrez & Manzanares 2010). In addition, the endocannabinoid system has received much attention in relation to the understanding of its functional significance in drug abuse and other neuropsychiatric disorders (Onaivi et al. 2008; Ishiguro et al. 2010). Several studies have suggested that both CB1r and CB2r have a role in alcohol addiction (Vinod et al. 2006; Ishiguro et al. 2007). The participation of CB1r has been demonstrated in several studies using genetic and pharmacologic approaches; deletion of CB1r reduced voluntary ethanol consumption (Hungund et al. 2003; Poncelet et al. 2003; Naassila et al. 2004; Thanos et al. 2005) and preference (Wang et al. 2003; Naassila et al. 2004; Thanos et al. 2005; Vinod et al. 2008). CB1KO mice presented significant higher blood ethanol concen- tration (BEC) after acute injection (Lallemand & De Witte 2005) but did not show ethanol withdrawal symptoms ORIGINAL ARTICLE Addiction Biology doi:10.1111/adb.12076 © 2013 Society for the Study of Addiction Addiction Biology