Investigation of the nature of the "stretch response" in suckling rats.

The "stretch response" in suckling rats has commonly been used as an index of milk ejection and milk intake. In the current study, we have compared the behavior of pups suckling on dams with either full, empty or ligated mammary glands. In response to an IV bolus of oxytocin, pups suckling on both full and empty nipples displayed the characteristic stretch behavior. Lack of milk release from the latter nipples was confirmed by lack of weight gain by the pups. In contrast to pups suckling on intact nipples, those suckling on ligated nipples did not display the stretch response. It is concluded that this behavior occurs in response to erection of the nipple and is not necessarily associated with milk release.