Rat pups are potent reinforcers to the maternal animal: Effects of experience, parity, hormones, and dopamine function

These experiments were designed to determine whether the parity difference in robustness of a maternal experience reflects a parity difference in the reinforcing value of pups and factors affecting pup reinforcement. Postpartum and virgin animals were exposed for 15, 30, or 60 min to a novel environment (either a horizontally or vertically striped box) in the presence or absence of pups (or food stimuli) over a 2-, 4-, or 8-day period and then tested for their box preference in a two-choice conditioned place preference paradigm. Postpartum animals preferred the pup-associated box to the alternative box; nulliparous animals did not. The reverse pattern was found when food, rather than pups, was paired with the distinctive environment. However, if nulliparous animals were induced to become maternal either by induction procedures or by hormonal manipulation, they also preferred the pup-associated box. The effect was most robust when animals were stimulated to be maternal by hormones. Finally, pup reinforcement shares properties with other types of reinforcers in being dependent on the functional integrity of the dopamine system. Postpartum animals treated with cis(Z)-flupentixol, a dopamine antagonist, do not develop a conditioned place preference when pups are the reinforcing stimulus. Taken together, these experiments show that pups are potent reinforcers to the maternal animal, regardless of how animals become maternal, but that the hormones of parturition may augment these effects. They show further that pup reinforcement, like food or drug reinforcement, is dopamine dependent.