Labour was induced by electrical stimulation of the infundibulum and median eminence in the conscious prepartum rabbit. The stimulus was applied for 20 s through a chronically implanted platinum—rhodium electrode and consisted of 1 mA biphasic pulses at 50/s.
The first visible signs of labour occurred 1·5–3·0 min after stimulation and activity usually ceased within 25 min. This was sometimes sufficient for the delivery of the entire litter. The litters ranged from 5 to 11 pups. Frequently, labour ceased before all the young were expelled, but labour was easily restarted by a further period of stimulation. There were cases in which labour was restarted on more than four occasions before parturition reached completion. Parturition was sometimes left in a suspended state for more than 24 h. The viability of the pups was not adversely influenced by the protracted nature of such parturitions. Stimulation was most effective at inducing labour when applied close to the predicted time of parturition and the treatment was almost totally ineffective when applied more than 48 h before the predicted time. Nest-building was not precipitated by the premature birth of the young and sometimes occurred a day or more after parturition.
The stimulation parameters used for the induction of labour caused large milk-ejection responses when applied to the same animals during lactation. Some of these milk-ejection responses were equal to the release of more than 100 mu. oxytocin.
These results suggest that the rapid expulsion of the pups during natural labour in the rabbit could be the result of a sudden and very large release of oxytocin.