Organization of the stretch response to milk in the rat fetus

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Abstract
Intraoral infusion of milk reliably elicits a stretch response in the rat fetus. The fetal stretch response appears isomorphic to the stereotypic behavior exhibited postnatally by suckling rat pups during milk ejection. Using a procedure that permits direct fetal observation, milk infusions were delivered to rat fetuses on Day 20 or 21 of gestation. Infusion of milk promoted systematic changes in fetal behavior that culminated in expression of the stretch response nearly 3 min later. Interruption of this behavioral sequence prevented stretching, suggesting that a period of behavioral reorganization after milk infusion is necessary but not sufficient for the stretch response to occur. The behavioral sequence initiated by the fetal rat's first exposure to milk is remarkably similar to the behavior exhibited by experienced infant rats at the nipple. © 1992 Wiley & sons. Inc.