Reinforcement value of sucrose measured by progressive ratio operant licking in the rat.

Progressive ratio (PR) schedules, which require increasing numbers of responses for successive reinforcements, are widely used to measure the reward value of foods, fluids, and drugs in operant lever-pressing tasks. The present study evaluated a PR operant licking task as a measure of sweet taste reward. In Experiment 1, food deprived rats were offered sucrose to drink on PR lick or fixed ratio (FR) lick schedules (30 min/day). In Experiment 2, nondeprived rats were offered sucrose to drink on PR or FR schedules and free access to water and food 23 h/day. In both experiments, the FR rats increased and then decreased their sucrose solution intake as concentration increased from 1% to 32% or 64%. The PR rats, in contrast, showed a near-linear increase in sucrose solution intake, lick rates, and break points (highest ratio completed) as a function of sucrose concentration. The PR rats drank less sucrose than did the FR rats although they emitted more total licks at the highest concentration tested. These results are similar to those reported with PR lever-pressing tasks. Thus, PR operant licking, which requires minimal training and equipment, is a useful alternate measure of fluid reward in rodents.