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ABSTRACT
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
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Levels of adrenal and peripheral plasma corticosterone are higher in the adult female than in the adult male rat and variations in oestrogen and androgen levels can alter corticotrophin and adrenocortical hormone metabolism (Troop, 1954; Coyne & Kitay, 1969; Ruhmann-Wennhold, Lauro & Nelson, 1970). Some studies conducted in male and female prepuberal animals at various ages have demonstrated no significant sexual differences in adrenal corticosteroids (Critchlow, Liebelt, Bar Sela, Mountcastle & Lipscomb, 1963); other investigators such as Troop (1954) and Yates, Herbst & Urquhart (1958) have suggested higher corticosteroid metabolism in males and in females respectively. During the first few days of life, sexual differentiation of the hypothalamus occurs, and the presence of androgens, as in the normal male or in the female injected with testosterone, induces a male pattern of gonadotrophin secretion. In contrast, low levels of androgens in the normal female or the male castrated at birth, induce
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ABSTRACT
At 8 and 13 days post partum, rats have the same total milk availability; yet, in response to suckling they release a greater amount of milk on day 13 than on day 8. Increased sensitivity to suckling in the more advanced lactators may result from a greater release of oxytocin or from changes in the mammary glands as lactation advances. The present study explores this latter possibility in anaesthetized dams at 8–9 and 13–15 days of lactation. Milk release and intramammary pressure were measured in anaesthetized dams in response to various doses of oxytoxin. Milk release was determined from the body weight gain of pups which had been fasted for 5 h before suckling on dams which had been isolated for 5 h. This parameter was significantly greater in 13- to 15-day lactators than in 8- to 9-day lactators over the range of oxytocin doses examined. In contrast, intramammary peak pressure and its dissipation time were significantly larger in the 8- to 9-day lactators than in the 13- to 15-day lactators. The compliance of the mammary glands was indirectly assessed at the two stages of lactation. When a constant pressure pulse was introduced into a cannulated gland, the resulting pressure peak was significantly greater in 8-day than in 13-day lactators, indicating a greater resistance in the former. Taken together, these results indicate that when endogenous oxytocin is inhibited (by anaesthesia) the greater milk release observed at the later stage of lactation in response to various doses of oxytocin may be due to a decline in mammary resistance as lactation progresses.
J. Endocr. (1987) 112, 379–385
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Numerous studies have provided evidence that intricate interactions exist among foetoplacental, ovarian and adrenal functions during pregnancy in rats (see Petropoulos, 1973). Such interactions are shown by: maternal adrenal hypertrophy and hyperfunction in pregnancy; longer survival of gravid than non-gravid rats after adrenalectomy; placental metabolic disturbances (Petropoulos, 1973), foetal adrenal hypertrophy, and decreased litter size and neonatal body weight after maternal adrenalectomy; premature placental ageing and foetal resorption after corticosterone administration to normal pregnant rats, but full-term, successful pregnancies in protein-deprived gravid rats given corticosterone. A question as yet unanswered, however, is whether the placenta secretes corticosterone into the general corticosterone pool of the gravid rat; this question led us to consider plasma corticosterone profiles of normal pregnancy, as well as the role of ovarian, adrenal and placental function in shaping these profiles. To accomplish these ends we used the scheme of classical endocrinological experiment, following the design of a
