The control of weaning was studied in rat pups aged 17-24 days. The influence of two hormones, thyroxine (T4) and corticosterone, and the effect of declining intestinal lactase activity were evaluated. Hypothyroidism was induced by administration of n-propylthiouracil and hyperthyroidism was induced by injection of T4. Hypothyroid pups failed to begin nibbling chow while littermates injected with T4 weaned normally. Two abnormalities resulting from hypothyroidism, hypothermia and stunted incisors, were not responsible for the lack of weaning in hypothyroid pups. Hyperthyroidism did not cause precocious weaning. Glucocorticoid levels were manipulated by both adrenalectomy (ADX) and administration of corticosterone. ADX pups exhibited a delayed pattern of weaning while both ADX pups injected with corticosterone and sham-operated pups weaned normally. Corticosterone injected before its normal developmental surge did not cause precocious weaning. Lactase activity, measured throughout these experiments, did not consistently reflect the degree of weaning progression. We conclude that 1) the hormones, T4 and corticosterone, are necessary for the onset of weaning, but neither is a sufficient stimulus to initiate weaning, and 2) low lactase activity does not initiate weaning.
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