Development of responses to an artificial nipple in the rat fetus: involvement of mu and kappa opioid systems.

Presentation of an artificial nipple to the rat fetus on E19, E20, or E21 of gestation promotes the expression of organized behavioral responses. Fetal responses include mouthing, licking, head-turning, and oral grasping of the nipple. Fetuses on E21 were more successful at grasping the nipple. Because all subjects were naive at the time of testing, this… (More)