Schedule induction of drug intake: differential responsiveness to agents with abuse potential

In previous research, two groups of rats exposed to a fixed time (FT) 1-min schedule of food-pellet delivery exhibited equal schedule-induced polydipsia for water vehicle and a 0.05 mg/ml solution of midazolam, respectively. Exposure to FT schedule values greater than 1 min (3 and 5 min) resulted in greater milliliter per pellet intakes of midazolam than of water, perhaps reflecting the reinforcing efficacy of the drug. To determine if drugs with unequivocal reinforcing efficacy display similar elevations at the larger FT values, solutions of cocaine and ethanol, but also of the equivocally reinforcing agents chlordiazepoxide and flurazepam, as well as water, were offered to groups of rats at concentrations isoacceptable under FT 1-min schedule-induction conditions. Under the FT 1-min schedule, separate groups of rats drank equal and excessive amounts of water, cocaine (0.15 mg/ml), ethanol (2.5% v/v), chlordiazepoxide (0.25 mg/ml) and flurazepam (0.175 mg/ml). Differential group milliliter per pellet intakes occurred under larger FT values: ethanol, cocaine and midazolam yielded greater values than water, chlordiazepoxide or flurazepam. Schedule-induced drug intake perhaps offers a method for evaluating the oral abuse potential of a drug under conditions which generate a variety of behavioral excesses and may potentiate intrinsic reinforcing efficacy.