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Following a 5-h isolation period, primiparous rats have the same milk supply on days 8/9 and 13/14 post partum, yet in response to suckling they release a greater amount of milk at the latter time. Inasmuch as stress is known to inhibit lactation and handling of the pups is stressful to the dams, the question arose as to whether separation from pups before nursing constitutes a greater stress for the dam at the earlier stage of lactation. This possibility was explored in the present study. As an index of stress, changes in plasma corticosterone were measured from chronically cannulated dams. In addition, the role of adrenal hormones and the peripheral sympathetic nervous system in the regulation of milk ejection under these experimental conditions was assessed following bilateral adrenalectomy and treatment with pentolinium, a ganglionic blocker which does not cross the blood–brain barrier. Corticosterone pellets were implanted subcutaneously following adrenalectomy in order to maintain proper lactation. From the results obtained, lactators demonstrated a greater increase in plasma adrenocorticosteroid levels in response to nursing on day 8/9 than on day 13/14 post partum, but milk yield was significantly less at the earlier than at the later stage of lactation. Adrenalectomy in conjunction with corticosterone replacement pellets did not alter milk supply or milk release. The subsequent treatment with pentolinium did not affect milk ejection. Pups still ingested less milk on day 9 than on day 13/14. It is suggested here that the isolation/suckling condition imposed on lactators may be more stressful earlier in lactation. Inasmuch as removal of the adrenal hormones along with ganglionic blockade did not modify the amount of milk ingested by the pups, it is concluded that the smaller milk release observed on day 9 post partum does not result from a direct suppression by the adrenal hormones or by the peripheral sympathetic nervous system on milk ejection.

J. Endocr. (1988) 118, 399–405