Env 223am

A pair of desktop computer speakers were placed in the bottom of the frame to provide auditory feedback. A pellet dispenser (ENV-203-1000; Med-Associates, St. Albans, VT, USA) was connected to the receiving raspberry pi via the PCB, located across the room. A Sonalert tone generator (ENV-223AM; 2900 Hz, 100 dB; Med-Associates) was also attached alongside the pellet dispenser.

Before the first self-administration training session, we gave the rats a 1-h magazine-training session, which began with the presentation of the white house light, followed by the noncontingent delivery of one pellet every 3 min. We used 45-mg food pellets (12.7% fat, 66.7% carbohydrate, and 20.6% protein; TestDiet 45-mg pellet, catalog #1811155). We then trained the rats to lever press for food during six 1-h sessions that were separated by 10 min for six consecutive days. The sessions began with the presentation of the white house light, followed 10 s later by the insertion of the food-paired active lever (right panel). The white house light remained on for the duration of the session and served as a discriminative cue for the palatable food. We trained the rats under a fixed-ratio-1 (FR1) 20-s timeout reinforcement schedule, where one lever press resulted in the delivery of five 45-mg palatable food pellets and the presentation of a 20-s discrete tone cue (ENV-223AM, Med Associates), during which additional lever presses were not reinforced but still recorded. At the end of each 1-h session, the white house light was turned off and the active lever was retracted. To match the number of discrete cue presentations to that of fentanyl (see below), we limited the number of food-reinforced deliveries to 12/h.