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Release of Oxytocin and Prolactin by Suckling in Rabbits throughout Lactation^*

ANNA-RIITTA FUCHS, LINDA CUBILE, M. YUSOFF DAWOOD and FINN STENER JØRGENSEN

Department of Obstetrics and Gynecology, Cornell University Medical College New York, New York 10021

Address requests for reprints to: Dr. Anna-Riitta Fuchs, Department of Obstetrics and Gynecology, The New York Hospital-Cornell Medical Center, 525 East 68th Street, New York, New York 10021.

Abstract

Plasma oxytocin and PRL were measured in serial samples of blood collected from lactating rabbits nursing five to seven (mean, six) young on a once-daily suckling regimen. Each suckling episode lasted 4.0 ± 1.1 (±SD) min on the average. Samples were obtained by means of an indwelling cardiac catheter before and 1, 3, 5, 10, 20, 30, and 60 min after suckling began. Measurements were performed at several stages of early, mid-, and late lactation.

Oxytocin levels rose to peak values during suckling and declined rapidly after suckling stopped. PRL levels, on the other hand, did not reach peak values until 1–5 min after suckling had stopped, at which time plasma PRL concentrations plateaued and, in early and midlactation, were sustained at peak levels for 2–3 h; in late lactation, PRL secretion was not sustained after suckling had ceased. Peak PRL levels were relatively constant throughout most of lactation, with no significant differences between groups until late in lactation, when peak levels fell rather abruptly from a mean of 74 ± 33.5 to 10.5 ± 13.3 (±SD) ng/ml around day 25 in spite of a constant number of young and constant suckling frequency. Suckling failed to elicit any PRL release on day 30, but the administration of fluphenazine, a dopamine antagonist, did cause a rise in plasma PRL. Oxytocin release increased with advancing lactation, rising, on the average, 40 pg/ml on day 2 and to 250 and 490 pg/ml in mid- and late lactation, respectively. Dopaminergic agonist and antagonist drugs given to the doe before the nursing episode did not influence oxytocin release in response to suckling.

Without a rise in plasma oxytocin, the young obtained no milk, but above a threshold level, there was no significant correlation between peak oxytocin levels and milk yield. When suckling failed to induce PRL secretion, milk secretion ceased rapidly in spite of copious oxytocin secretion. The failure of suckling to induce PRL release in late lactation, therefore, appears to be an important factor in the cessation of lactation. (Endocrinology 114: 462, 1984)

Footnotes

^* Part of this material has been presented at the 61st Annual Meeting of The Endocrine Society, 1979. This work was supported in part by the O. W. Caspersen Foundation.

Present address: Department of Obstetrics and Gynecology, The Abraham Lincoln School of Medicine, University of Illinois at the Medical Center, 840 South Wood Street, Chicago, Illinois 60612.

Received March 16, 1983.

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