Mice were trained to self-administer a 20% alcohol solution in operant-conditioning chambers (ENV-307A, Med Associates Inc., St Albans, VT, USA) prior to starting alcohol exposure. Briefly, the chambers contained two nose-poke holes, located on one wall of the chamber 3 cm above a grid floor, fitted with a photocell and a yellow cue light, and a steel dish into which reinforcers were delivered from a syringe pump (Thomsen and Caine, 2005 (link); Bornebusch et al., 2019 (link)). All chambers were individually enclosed in sound-attenuating boxes equipped with a light and a ventilation fan. A nose-poke in the active hole triggered a syringe pump to deliver 30 μL of the reinforcer and the cue light to turn on. Nose-pokes in the inactive hole were recorded but had no scheduled consequences. Training sessions consisted of different solutions, starting with Nutridrink Compact Protein Vanilla (Nutricia, Amsterdam, Netherlands), then Calogen unflavored fat emulsion (Nutricia, Amsterdam, Netherlands), Calogen with 10% alcohol, Calogen with 20% alcohol and eventually water with 20% alcohol.
Training sessions used a fixed ratio (FR) 1 schedule of reinforcement, with a post-reinforcer timeout of 20 s, running for 1 h daily until criteria were met, i.e., a minimum of 20 reinforcers were earned within one session. Experimental sessions consisted of a progressive ratio (PR) schedule of reinforcement running for a maximum of 6 h per day. PR sessions terminated when no reinforcers were administered for 1 h (i.e., 1 h limited hold), at which time the number of reinforcers earned was recorded as the breaking point and the session ended. PR sessions consisted of a PR1 training period (response requirement 1, 2, 3 etc.), then, an exponential progression starting at 3 responses and increasing by a factor of 0.115 log units (response requirement 3, 4, 6, 7, 10, 13, 16, 21, 28, 36 etc.) which was used as the experimental schedule of reinforcement. The reinforcer-delivery cup was examined at the end of the sessions for any solution left unconsumed; in all the PR experimental sessions, all solution was consumed.
Training sessions used a fixed ratio (FR) 1 schedule of reinforcement, with a post-reinforcer timeout of 20 s, running for 1 h daily until criteria were met, i.e., a minimum of 20 reinforcers were earned within one session. Experimental sessions consisted of a progressive ratio (PR) schedule of reinforcement running for a maximum of 6 h per day. PR sessions terminated when no reinforcers were administered for 1 h (i.e., 1 h limited hold), at which time the number of reinforcers earned was recorded as the breaking point and the session ended. PR sessions consisted of a PR1 training period (response requirement 1, 2, 3 etc.), then, an exponential progression starting at 3 responses and increasing by a factor of 0.115 log units (response requirement 3, 4, 6, 7, 10, 13, 16, 21, 28, 36 etc.) which was used as the experimental schedule of reinforcement. The reinforcer-delivery cup was examined at the end of the sessions for any solution left unconsumed; in all the PR experimental sessions, all solution was consumed.
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