The rats were initially trained in daily (Monday - Friday) 15 min sessions to press one of two levers under a fixed-ratio 1 (FR1) schedule of reinforcement in which each lever press resulted in a food pellet delivery. The response requirement was gradually increased to FR10. During the next few sessions, the rats were reinforced only for pressing the opposite lever until they pressed reliably under FR10 contingencies. Rats were then injected i.p. with 3 mg/kg THC or vehicle 30 min prior to the start of the session. For each rat, the experimenter designated one lever as correct (and pressing it resulted in pellet delivery) following THC administration and the other as correct following vehicle administration. The lever on which the rats were initially trained and on which they acquired the lever-press response was designated as the “vehicle-correct” lever. Alternation of THC and vehicle injections followed a two-monthly cycle (Month #1: TVVTV, VTTVT, VTVTV, TVTVT; Month #2: VTTVV, TVTVT, TVVTT, VTVTV; in which T=THC, V=vehicle). Lever pressing produced pellet delivery only on the injection-appropriate lever during a training session. Incorrect presses reset the response requirement on the correct lever.
