The Milk-Ejection Reflex of the Rat: An Intermittent Function Not Abolished by Surgical Levels of Anaesthesia

SUMMARY Milk ejection (ME) was studied in the lactating rat by the measurement of intramammary pressure, milk yield and behaviour. An intermittent pattern of ME was observed, both in conscious rats and in rats anaesthetized with tribromoethanol, pentobarbitone sodium and ethyl carbamate. Milk ejection was never seen as a reflex response with the onset of the suckling stimulus; the latency to the first ME varied from 10 min in the conscious rat to 60 min during deep anaesthesia. Thereafter, ME occurred at regular intervals ranging from 3 to 5 min under optimal conditions to 15 min during less favourable circumstances, though the young were content to suckle continuously for several hours irrespective of whether they obtained milk. Each ME was associated with an abrupt and uniform rise in intramammary pressure and simulation of the individual ME was obtained by a rapid i.v. injection of 1 mu. oxytocin. The pups reacted immediately to the rise in pressure with a synchronized extensor reaction and a vigorous increase in suckling which lasted for about 10 s. There was, however, no apparent change in the suckling stimulus to account for the release of endogenous oxytocin some 10–20 s prior to these events. Milk ejection was a reflex phenomenon, nonetheless, for the removal of the pups abolished the intermittent release of oxytocin. The rise in intramammary pressure was associated directly with the ejection of the milk and the pups abruptly increased in weight. Yields of 10–12 g were obtained from both conscious and anaesthetized rats after 18 h of separation from their young. Most of this milk was expressed in the first ten milk ejections. Mammary distension and the size of the litter were important parameters determining the pattern of ME. The incidence of ME was appreciably lower in unseparated rats whether anaesthetized or not and in separated rats after prolonged suckling or artificial removal of the milk. A similar reduction was encountered when the number of suckling pups was changed from ten to five. The physiological principles of the ME reflex in the rat are discussed and a theoretical model is presented.