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 improved performance occurred in the absence of explicit experience with the nipple or specific practice (grasping the nipple). Manipulation of kappa and mu opioid activity with selective agonist drugs (U50,488 and DAMGO, respectively) altered fetal responsiveness to the nipple. U50,488 generally disrupted appetitive responses and promoted aversive reactions to the artificial nipple, whereas DAMGO increased responsiveness (licking and oral grasping), especially on E21. These findings suggest that both the mu and kappa systems may play functional roles in regulating neonatal behavior at the nipple by initiating and terminating a suckling bout.