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

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 intravenouslyintravenously administered oxytocin in mice was 128 μU±49 μU (X̄±S) and in rats 205 μU±77 μU. Retrograde injection into the carotid artery delivers oxytocin to the upper abdominal mammary glands. Here the threshold dose was 72 μU ± 29 μU in mice and 135 μU ± 42 ymU 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 μU ± 14 μU has been established. No oxytocin was measurable in venous blood of 12 normal nonpregnant women. (Endocrinology81: 711, 1967)