Effect of intrahypothalamic implantation of cortisone acetate on the onset of circadian corticosterone rhythm in neonatal female rats.

Subcutaneous administration of corticosteroids to neonatal rats has been reported to delay the onset of the circadian corticosterone rhythm. Micropellets of a cortisone acetate(CA)-paraffin mixture or paraffin alone were implanted intrahypothalamically or subcutaneously in 2-day-old female rats. 24- or 48-hour patterns of blood corticosterone were obtained serially in individual rats at 21, 28, 35, 56 and 120 days of age. Sham(paraffin)-implanted rats and those bearing CA pellets subcutaneously showed a well-defined circadian corticosterone rhythm at day 28. CA implantation into the anterior hypothalamus, however, caused a delay in the onset of the circadian corticosterone rhythm by 1 week together with a diminished amplitude. In rats with CA pellets in the medial basal hypothalamus, the corticosterone rhythm was not fully established even on day 56, but was evident in most animals on day 120. It is suggested that neonatal CA treatment exerts its effect by inhibiting the maturation of the efferent neural pathways carrying circadian signals from the suprachiasmatic nuclei.