A modified adjusting delay task to assess impulsive choice between isocaloric reinforcers in non-deprived male rats: effects of 5-HT2A/C and 5-HT1A receptor agonists

Existing animal models of impulsivity frequently use food restriction to increase subjects' motivation. In addition, behavioral tasks that assess impulsive choice typically involve the use of reinforcers with dissimilar caloric content. These factors represent energy-homeostasis limitations, which may confound the interpretation of results and limit the applicability of these models. This study was aimed at validating face and convergent validities of a modified adjusting delay task, which assesses impulsive choice between isocaloric reinforcers in ad libitum fed rats. Male Wistar rats (n = 18) were used to assess the preferredness and reinforcing efficacy of a "supersaccharin" solution (1.5% glucose/0.4% saccharin) over a 1.5% glucose solution. A separate group of rats (n = 24) was trained in a modified adjusting delay task, which involved repeated choice between the glucose solution delivered immediately and the supersaccharin solution delivered after a variable delay. To pharmacologically validate the task, the effects of the 5-HT2A/C receptor agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane [(+/-)-DOI] and the 5-HT1A receptor agonist (+/-)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide [(+/-)-8-OH-DPAT] on impulsive choice were then evaluated. Supersaccharin was highly reinforcing and uniformly preferred over the glucose solution by all subjects. Rats quickly learned the task, and impulsivity was a very stable and consistent trait. DOI and 8-OH-DPAT significantly and dose dependently increased impulsive choice in this modified adjusting delay task. We validated a rodent task of impulsive choice, which eliminates typical energy-homeostasis limitations and, therefore, opens new avenues in the study of impulsivity in preclinical feeding and obesity research.