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, h as been suggested to be under the preferential control of central sero tonin (5-HT) function. The present study examined the effects of the a cute administration of drugs which directly or indirectly alter 5-HT t ransmission on tolerance to delay of reward in rats subjected to a pro cedure of discrete-trial choice in an operant chamber. Different group s of rats were trained to choose between two levers giving access to r einforcers differing in both magnitude and delay: one food pellet, del ayed by 0 or 5 s, vs. five pellets delivered after a prereinforcer int erval fixed at either 15, 30, 45, or 60 s, depending on the experiment s. The learning curves indicated that rats were able to adjust their c hoice strategy precisely according to various factors: the respective duration of the delays before the small and large rewards, the immedia cy of the small reward delivery, and the lengthening of the trials by a postreinforcer delay (or intertrial interval). Whatever the experime ntal parameters and stage of the learning, an acute administration of drugs able to reduce 5-HT neuronal activity (benzodiazepines; 5-HT1A r eceptor partial agonists: buspirone and MDL 73005EF) or enhance 5-HT t ransmission (5-HT reuptake inhibitors: indalpine and zimelidine; 5-HT1 A receptor full agonist: 8-OH-DPAT) failed significantly to alter choi ce strategy (decreased or increased preference for the large but delay ed reward, respectively), as they did in other situations such as a T- maze procedure. Only d-amphetamine (0.5 mg/kg), on one occasion, signi ficantly 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 rei nforcement. These results indicate that the present operant paradigm i s not sensitive to acute modifications in the internal state of the an imals and in the reward contingencies, and therefore is not useful to evaluate tolerance to delay and variations in impulsiveness in rats.