The effect of total hypothalamic deafferentation on the nycthemeral rhythm of plasma corticosterone concentration was studied in rats with in situ or heterotopically transplanted pituitaries. Ten pituitaries from 21-day-old donors were transplanted in 120-g male recipients. Hypophysectomy was performed four weeks following transplantation. One week later the basal hypothalamus was surgically isolated. Post-transplantation AM and PM levels of plasma corticosterone were not different from those of unoperated controls. Following hypophysectomy there was a reduction in the amplitude of the plasma corticosterone rhythm. Surgical isolation of the basal hypothalamus, in both the heterotopic and in situ preparations, abolished the AM-PM differences of plasma corticosterone concentration. It is concluded that the nycthemeral rhythm of ACTH secretion of both in situ and heterotopic pituitaries requires the integrity of neural connections between the basal hypothalamus and the rest of the brain.
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