Dual Heroin and Food Self-Administration in Rats

Rats were surgically implanted with an intravenous jugular catheter as previously described [6 (link)]. After recovery, rats were trained to self-administer food and heroin in tandem over daily (weekday) sessions inside standard rat operant chambers (Med Associates, St. Albans, VT, USA). During the first 30 min of each session, the food (right) lever and cue (3.5 kHz tone, 5 sec) were available, followed by a 10 min time-out with levers retracted. Thereafter, the heroin (left) lever and cue (light above lever, 5 sec) were available for 2 h. Training began on a fixed-ratio 1 (FR1) schedule of reinforcement. Cues for each reward were initiated simultaneously with reward delivery onset, and levers retracted during cue presentation. After 8 sessions, training progressed through 8 additional FR steps (FR3, FR8, FR20, FR50, FR125, FR313, FR783, FR1958) with at least 1 day on each FR. To prevent infection and catheter occlusion, respectively, cefazolin and taurolidine citrate solution (TCS) were administered after each self-administration session. Catheter patency was periodically verified using methohexital sodium (1 mg/0.1 ml, i.v.). Five rats were excluded from the final dataset due to defective catheters.