2.2. Intermittent-access 20% ethanol drinking paradigm

Prior to operant training, rats received homecage intermittent-access to 20% ethanol (EtOH) (Carnicella et al., 2014; Moorman and Aston-Jones, 2009; Simms et al., 2008; Wise, 1973). Rats were given access to 20% EtOH during three 24-h sessions per week (Mondays, Wednesday and Fridays) with 24 or 48 h-ethanol deprivation between EtOH access days. During 24-h session, each rat received 40 ml of 20% EtOH in a 50 ml graduated plastic cylinder with ball point stainless-steel drinking spout. After 24 h, excluding the Friday session, the EtOH bottle was replaced with a similar bottle with 40 ml of tap water for the next 24 h. Over the weekend, rats did not receive either bottle. Rats were given intermittent-access to 20% EtOH for 12 sessions (4 weeks). 24-hour drinking session began during their dark cycle (from ~10:00 A.M.). Throughout this period, rats had access to their home-cage water bottle ad libtum. The weight of each rat was measured before beginning the 24-h EtOH drinking session in order to calculate the grams of EtOH consumed per kilogram of body weight. The amount of EtOH and water consumed was recorded after every 24-h session. EtOH solutions were prepared from 95% (v/v) EtOH (Fisher Scientific, Pittsburgh, PA) in tap water.