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.

Oral capture and grasping of an artificial nipple by rat fetuses.

Although born blind and deaf, newborn rats exhibit a remarkable capacity to recognize and gain access to the nipples of the lactating mother. However, it is well-known that full-term rat neonates will not attach to an artificial nipple. In the present study, an artificial nipple fashioned from soft vinyl was presented to rat fetuses from E17-E21 of gestation. Fetuses showed side-to-side head movements that resulted in oral capture of the nipple on E18 and exhibited a direct nipple-grasping response from E19 through term. Frame-by-frame analysis of videotape records of E21 rat fetuses revealed that tactile contact with the artificial nipple elicited mouthing, licking directed at the nipple, forelimb treadling, and grasping of the nipple. Fetuses also exhibited components of aversive behavior, including facial wiping and head turning, that appeared to terminate oral contact with the nipple. Morphine pretreatment reduced the expression of aversive responses and promoted licking and grasping of the artificial nipple. In addition to documenting the prenatal ontogeny of this important neonatal behavior, these findings imply a role for endogenous opioids in the newborn rat's first suckling episode.