Oxytocin-Induced Stretch Reaction in Suckling Mice and Rats: A Semiquantitative Bio-assay for Oxytocin

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Abstract
Pups suckling on lactating mice or rats which have been anesthetized with ethanol show a unique reaction after the intravenous or intra-arterial injection of oxytocin into the mother. Characteristically, within 7 to 18 sec after the Intravenous injection or within 2 to 8 sec. after the intra-arterial injection of oxytocin, the suckling animals develop a stretch position. The hind legs elevate the posterior part of the body, while the anterior extremities are pushed against the abdomen of the mother. During this reaction (3-5 sec), the pups strain maximally upon the nipples. The mean dose eliciting this reaction (mean threshold dose) for intravenously administered oxytocin in mice was 128 [mu]U [plus or minus] 49 [mu]U ([image] [plus or minus] s) and in rats 205 [mu]U [plus or minus] 77 [mu]U. Retrograde injection into the carotid artery delivers oxytocin to the upper abdominal mammary glands. Here the threshold dose was 72 [mu]U [plus or minus] 29 [mu]U in mice and 135 [mu]U [plus or minus] 42 [mu]U in rats. Retrograde injection into the femoral artery (lower abdominal mammary glands) further increases the sensitivity, and for rats a mean threshold dose of 30 [mu]U [plus or minus] 14 [mu]U has been established. No oxytocin was measurable in venous blood of 12 normal nonpregnant women.