QUANTITATIVE ASPECTS OF RELEASE OF OXYTOCIN BY SUCKLING IN UNANAESTHETIZED RABBITS

Abstract

ABSTRACT A method of estimating oxytocin output following physiological stimuli is described which is applicable to experiments in conscious, undisturbed and unrestrained animals. The method consists of recording the uterine responses in puerperal rabbits with a permanently inserted intrauterine balloon as an index of neurohypophysial activation. Previous work has been confirmed on the essential role of oxytocin in milk ejection in rabbits. Only negligible amounts of milk are obtained by the young without any endogenous or exogenous oxytocin, but normal release of oxytocin can occur in spite of diminished or completely inhibited milk flow. The amount of oxytocin released during suckling depends on the strength of the stimulus in a characteristic fashion, and the strength of stimuli again depends both on the number of young suckling simultaneously and on their physical maturity. In the early puerperium (up to 3–4 days post-partum) each suckling young evokes a response comparable to that of 0.5–1.0 mU of oxytocin injected intravenously, whether suckling alone or together with its litter-mates. Only a few grams of milk are removed by each young at that time. After this period, the suckling of 1 or 2 young separately still elicits the release of only a small amount of oxytocin (1–2 mU), but during suckling of a whole litter from 50 to 100 mU are liberated, which corresponds to 10–15 mU for each young. The full milk yield can only be obtained by each young while suckling simultaneously with the whole litter. The amount of oxytocin released is independent of the duration of suckling, of the quantity of milk available, and of the milk flow. Once lactation has been established, the oxytocin output during suckling on an »once-daily« regime remains fairly constant at least for a large part of the lactation period. It is possible that at the end of the lactating period, the amount secreted during suckling is increased to about 250 mU or even more.