Effects of psychotropic drugs on rat responding in an operant paradigm involving choice between delayed reinforcers.

Preference for immediate reward, taken as an index of impulsiveness, has been suggested to be under the preferential control of central serotonin (5-HT) function. The present study examined the effects of the acute administration of drugs which directly or indirectly alter 5-HT transmission on tolerance to delay of reward in rats subjected to a procedure of discrete-trial choice in an operant chamber. Different groups of rats were trained to choose between two levers giving access to reinforcers differing in both magnitude and delay: one food pellet, delayed by 0 or 5 s, vs. five pellets delivered after a prereinforcer interval fixed at either 15, 30, 45, or 60 s, depending on the experiments. The learning curves indicated that rats were able to adjust their choice strategy precisely according to various factors: the respective duration of the delays before the small and large rewards, the immediacy of the small reward delivery, and the lengthening of the trials by a postreinforcer delay (or intertrial interval). Whatever the experimental parameters and stage of the learning, an acute administration of drugs able to reduce 5-HT neuronal activity (benzodiazepines; 5-HT1A receptor partial agonists: buspirone and MDL 73005EF) or enhance 5-HT transmission (5-HT reuptake inhibitors: indalpine and zimelidine; 5-HT1A receptor full agonist: 8-OH-DPAT) failed significantly to alter choice strategy (decreased or increased preference for the large but delayed reward, respectively), as they did in other situations such as a T-maze procedure. Only d-amphetamine (0.5 mg/kg), on one occasion, significantly reduced preference for the larger reward. The choice strategy was also insensitive to acute changes in experimental parameters such as a reduction in delay or increase in the magnitude of the large reinforcement. These results indicate that the present operant paradigm is not sensitive to acute modifications in the internal state of the animals and in the reward contingencies, and therefore is not useful to evaluate tolerance to delay and variations in impulsiveness in rats.