Conditioning chamber

All behavioral procedures were conducted in conditioning chambers (15.9 × 14 × 12.7 cm; Med Associates), each enclosed within a sound-attenuating and light-resistant cubicle. The conditioning chamber was fitted with a recessed magazine situated in the center of one side wall, which dispensed 10% sucrose solution serving as the unconditioned stimulus (US). An infrared beam detected head entries into the magazine. The house light was situated in the side panel and was on for the duration of each training or test session. A mechanical relay served as an auditory (click) conditioned stimulus (CS; Med Associates). Initiation and running of behavioral protocols, including the recording of head entries into the food magazine, was performed using Med-PC IV (Med Associates; RRID:SCR_012156).

Before the onset of PCA testing, rats were handled daily for 3 d and given approximately seven banana-flavored sucrose pellets (45 mg; BioServ). Rats were tested in conditioning chambers (20.5 × 24.1 cm floor area, 20.2 cm high; MED Associates Inc.). Each chamber contained a food magazine port located 2.5 cm above the floor in the center of the intelligence panel, a red house light located on the wall opposite the food magazine port (on throughout training sessions), and a retractable lever (Med Associates) located 2.5 cm to the left or right of the food receptacle and 6 cm above the floor. This retractable lever was illuminated when extended with a white LED light placed inside the lever house. For a lever press to be recorded, a force of ∼15 g was needed. The pellet dispenser (Med Associates) delivered one 45-mg banana-flavored sucrose pellet (Bio-Serv) into the food magazine port at a time. A head entry was recorded each time a rat broke the infrared photobeam located inside the food magazine port. Each conditioning chamber was placed in a sound-reducing enclosure and a ventilating fan served as background noise. Data collection was controlled by Med-PC IV Behavioral Control Software Suite.

120 min behavioral sessions were conducted >4 weeks post-surgery in conditioning chambers (Med Associates Inc.) contained within sound-attenuating cubicles. Session start was indicated to the rat by the illumination of a chamber light and the onset of low-volume white noise (65 dB) to mask external sounds. Two nosepoke ports, designated “active” and “inactive”, were positioned on the left chamber wall; each had three LED lights at the rear. A response at the active nosepoke port resulted in optical stimulation (20 pulses, 5 ms duration, 20 Hz, 473 nm) on a fixed-ratio 1 schedule, with the exception that a new stimulation train could not be earned until any ongoing train had finished. The LED lights in the recess of the active port were illuminated concurrent with stimulation. We chose to include a response-contingent cue in our experimental design because such cues have been show to facilitate robust operant responding over long periods in drug self-administration studies [24] (link). Responses at the inactive nosepoke port were recorded but had no consequence. During the first training session, both nosepoke ports were baited with a crushed cereal treat to facilitate initial investigation.

Operant training and testing took place in conditioning chambers (24.0 × 31.0 × 21.0 cm high; Med Associates). The front wall of the chamber contained the food opening and two retractable levers (4.5 × 0.1 cm) that extended 1.8 cm into the chamber. Each lever was located 1.0 cm from the side wall and 2.0 cm from the floor. Only the left lever was used in this experiment. A rectangular food opening (5.0 × 4.5 cm) was located 9.5 cm from the side wall and 0.5 cm from the floor. The front wall also contained two white cue lights above the levers and a tone generator (2 kHz, 15 dB over ambient noise) above the left lever. Only the white cue light above the left lever was used in this experiment. A red house light (24-V) was located in the back wall and 2.0 cm below the ceiling. Operant chambers were contained in sound-attenuating enclosures. A ventilating fan masked noises from outside the enclosures.