Further Studies on Inhibition of Milk Ejection by Administration of Oxytocin to the Mouse

Abstract

Oxytocin in doses of 0.25, 0.5 and 1 I. U. was injected s. c. twice daily for 5 days from day 4 of lactation and the milk ejection response was measured on the last day of injection. The difference of litter weight during 1 hr nursing after 8 hr isolation from the mother was considered as the amount of naturally ejected milk. To obtain the residual milk the litters were allowed to suckle for an additional 30 min after oxytocin in the corresponding doses in each group was injected. Two control groups were similarly injected with 0.1ml of 0.5% chloretone solution twice daily, and to obtain the residual milk single injections of 1 I.U. oxytocin and 0.1ml of saline were given, respectively. As the levels of oxytocin increased, the growth rate of the litter decreased and the % of residual milk increased. In the 2nd experiment, single injections of oxytocin in varying doses from 0.125 to 1 I.U. could recover the % of residual milk substantially to the same extent in groups in which the natural milk ejection had been inhibited by treatments with 1 I.U. of oxytocin twice daily for 5 days. It was shown in the 3rd experiment that in the course of repeated injections of oxytocin, the litter could obtain the milk for only a short period after every oxytocin injection, but they became unable to obtain the milk and lost weight during any other period despite the fact that they vigorously suckled the teats. These results indicate that exogenously administered oxytocin inhibited lactation, primarily through inhibition of the milk ejection in response to normal suckling stimulus.