Progressive Ratio Operant Behavior

Mouse operant boxes (ENV-307W, Med Associates Inc., USA) fitted with 2 levers, a cue light above the active lever (AL), a house light, a speaker and a liquid receptacle were used. Throughout all sessions, when the number of AL presses to complete a ratio was reached, the house light and a tone were presented for 5sec, after which a sucrose reward was delivered for 2sec (38ul, 20% w/v sugar solution in tap water). During the training phase, mice were food restricted to ~90% of their original body weight. The first day, mice were habituated for 15min to an operant box, in which we placed a droplet of sugar solution (20% w/v) in the receptacle. The next day, operant training started with a fixed ratio (FR) 1 paradigm for 30min/session. Once mice learned to press on the active lever >20x and <10% on the inactive lever, mice were switched to FR3 (30min/session) and then FR5 (60min/session). Then, once >60 rewards were earned and <10% of the presses were made on the inactive lever, training was switched to the progressive ratio (PR, 60min/session) and mice were returned to ad libitum feeding. PR schedule was based on the formula: 5*e^(x*0.2)-5, rounded to the nearest integer, where x is the position in the ratio sequence (30). Testing with saline/CNO started when PR performance appeared stable, i.e. over 3 days of training no more than ±1 reward from average and no incremental increase or decrease. Operant training and testing were performed during the first 3 hours of the dark phase. Injections were given 30min prior to the start of the PR task. With this test we measured the amount of active lever presses made during 60min of the PR task.