Aspects of the neuronal and endocrine components of reflex milk ejection in conscious rabbits

Nursing behaviour and reflex milk ejection were studied over 257 suckling periods in 26 Californian rabbits. Detailed plasma profiles of oxytocin were obtained for 161 suckling periods in 16 animals. Plasma oxytocin was detected by radioimmunoassay in serial samples of 0.2 ml blood collected during nursing. Oxytocin titres were below the lower limit of the assay (5 pmol/l) before suckling, and started to rise at a rate of 2.8 +/- 0.8 (S.D.) pmol/l per s 10-30 s after the onset of suckling. Peak levels of hormone were 345 +/- 113 pmol/l and were attained towards the end of nursing. In 33 of these experiments simultaneous records of activity from oxytocin neurones were taken whilst chronically sampling the blood. Each neurone gave an average of seven bursts of neurosecretory activity in suckling. Onset of this bursting pattern of discharge began 5-24 s before the rise in plasma oxytocin was detected. Oxytocin neurone activity alone was monitored in a further 55 suckling periods in eight rabbits. A marked relationship between the duration of suckling period, milk yield, peak oxytocin levels and the length of neurosecretory bursts was demonstrated over the course of lactation. All four parameters increased in parallel from day 1 to reach maximal values on days 15-20 of lactation then started to wane until the doe ceased nursing her litter on days 25-27 of lactation.