Reduced ethanol response in the alcohol-preferring RHA rats and neuropeptide mRNAs in relevant structures Marc Guitart-Masip, 1,2 Lydia Gime ´ nez-Llort, 1 Albert Ferna ´ ndez-Teruel, 1 Toni Can ˜ ete, 1 Adolf Toben ˜a, 1 Sven Ove O ¨ gren, 3 Lars Terenius 2 and Bjo ¨ rn Johansson 2,3 1 Medical Psychology Unit, Department of Psychiatry and Forensic Medicine, Neuroscience Institute, Autonomous University of Barcelona, 08193 Bellaterra, Barcelona, Spain 2 Department of Clinical Neuroscience, Karolinska Institutet, KS CMM L8:01, SE-171 76 Stockholm, Sweden 3 Department of Neuroscience, Karolinska Institutet, KS CMM L8:01, SE-171 76 Stockholm, Sweden Keywords: cholecystokinin, dynorphin, enkephalin, hole-board test, motor activity Abstract Roman rat strains, genetically selected for high (RHA) or low (RLA) active avoidance acquisition in the two-way shuttle box, differ in dopaminergic activity. These two strains appear to be a valid laboratory model of divergent sensation ⁄ novelty and substance-seeking profiles. RHA rats show higher ethanol intake and preference than do RLA rats, and it was suggested that RHA rats are more tolerant than RLA to the effects of alcohol. In the hole-board test, we found that the non-alcohol-preferring RLA rats showed enhanced responsiveness to the stimulatory effects of intraperitoneal administration of 0.25 g ⁄ kg ethanol when compared with RHA rats. In situ hybridization analysis showed higher levels of preprodynorphin in the accumbens shell and higher levels of preproenkephalin in the cingulate cortex in RHA rats. RLA rats showed higher levels of enkephalin gene transcripts in restricted areas of the dorsal striatum. Finally, differences in cholecystokinin gene transcript, suggestive of a different arrangement of certain interneurons, were found in different cortical areas. The differences in peptide gene expression found between the two strains might reflect the differences in alcohol preference and sensitivity. RHA rats may have more predictive value than other rodent alcoholism models, as high initial tolerance to ethanol is a risk factor for alcoholism in humans. Introduction As in other addictive behaviours, ethanol (EtOH) consumption has been linked with the mesolimbic dopaminergic system that marks incentive salience or predicts reward (reviewed by Gonzales et al., 2004). EtOH, like most abused drugs, increases dopamine (DA) neurotransmission in the nucleus accumbens and other areas of the mesolimbic system (Imperato & Di Chiara, 1986). However, EtOH interacts with many other neurotransmission systems. The enhanced DA neurotransmission seems to be mediated by the opioid system (reviewed by Oswald & Wand, 2004). Activation of l and d receptors on c-aminobutyric acid (GABA) interneurons in the ventral tegmental area (VTA) enhances dopaminergic neuron firing activity (Johnson & North, 1992). Simultaneously, a large body of evidence has shown that DA regulates the expression of opioid peptides in striatal projecting neurons (Nylander & Terenius, 1987; reviewed by Angulo & McEwen, 1994). Another neuropeptide that interacts with DA and has been related to addictive behaviours is cholecystokinin (reviewed by Rotzinger & Vaccarino, 2003). Cholecystokinin reaches the striatum from pyramidal neurons and dopaminergic neurons (reviewed by Ho ¨kfelt et al., 2002). An antagonistic effect of cholecystokinin on the D 2 receptor through preprocholecystokinin (CCK) b receptor activation has been demonstrated both at the behavioural and cellular levels (Fuxe et al., 1995). Current theories and experimental evidence consistently suggest a relationship among enhanced mesolimbic dopaminergic transmission and higher novelty ⁄ incentive-seeking and drug-seeking behaviour (e.g. Bardo et al., 1996; Zuckerman, 1996). In this regard, the sublines of Roman high- (RHA) and low-avoidance (RLA) rats, psychogenet- ically selected for, respectively, rapid vs. extremely poor two-way active avoidance acquisition (Driscoll & Ba ¨ttig, 1982), appear to be a valid laboratory model of divergent novelty- and substance-seeking profiles, as well as of differential central DAergic activity in a wide range of experimental situations (reviewed by Driscoll et al., 1998). Thus, compared with RLA ⁄ Verh rats, RHA ⁄ Verh rats show: (i) higher levels of exploratory behaviour in tests of novelty seeking (Ferna ´ndez- Teruel et al., 1992, 1997a, 2002; Escorihuela et al., 1999); (ii) higher preference for alcohol (Driscoll et al., 1990; Razafimanalina et al., 1996; Corda et al., 2001), as well as saccharin and quinine solutions (Razafimanalina et al., 1996; Ferna ´ndez-Teruel et al., 2002); (iii) stronger mesocortical and mesolimbic dopaminergic responses to, respectively, stress (D’Angio et al., 1988; Giorgi et al., 2003) and addictive substances, including EtOH (Giorgi et al., 1997, 2005; Corda et al., 2001). Moreover, RHA ⁄ Verh rats are less sensitive than RLA ⁄ Verh rats to the hypnotic effects of alcohol (Ferna ´ndez-Teruel et al., 1997b). On the premise that a low sensitivity to EtOH is associated with a higher risk for alcoholism in humans (Schuckit, 1994), we investi- gated whether the stronger EtOH preference of RHA vs. RLA rats may be related to a lower sensitivity of the RHA strain to the acute effects of EtOH. The hole-board test of novelty seeking was used, as this test Correspondence: Dr B. Johansson, 3 Department of Neuroscience, as above. E-mail: bjorn.johansson@neuro.ki.se Received 1 May 2005, revised 3 November 2005, accepted 10 November 2005 European Journal of Neuroscience, Vol. 23, pp. 531–540, 2006 doi:10.1111/j.1460-9568.2005.04556.x ª The Authors (2006). Journal Compilation ª Federation of European Neuroscience Societies and Blackwell Publishing Ltd