Inhibition of Milk Ejection by Exogenous Oxytocin in Lactating Rats

Abstract

Summary

The effect of different doses of exogenous oxytocin ranging from 100 to 500 m-u on the milk ejection reflex was studied in lactating rats which were isolated for 9 hr from their litters. The litters were then returned to the injected mothers and allowed to nurse for 30 min. The ip injection of 250 or 500 m-u into lactating rats 90 min before the suckling period significantly blocked milk ejection. A small dose of oxytocin injected immediately before nursing to the latter group induced a normal milk ejection indicating that the blocking effect was not due to a lack of mammary gland response. A dose of 100 m-u did not block milk ejection. When oxytocin was injected 60 min before suckling, only 500 m-u decreased the amount of milk obtained by the young whereas the 250 m-u dose was ineffective. On the contrary when the 500 m-u of oxytocin was injected 30 min before nursing milk yield was significantly increased. This was caused by the milk ejecting activity of the exogenous oxytocin since the injection of sodium pentobarbital to block the milk ejection reflex did not prevent the pups from obtaining milk 30 min after oxytocin. Antidiuretic hormone at a dose of 50 m-u did not modify milk ejection, but a 500 m-u dose inhibited it. It is suggested that oxytocin may regulate its own release in lactating rats.