Return of diurnal plasma corticosterone rhythm long after frontal isolation of the medial basal hypothalamus in the rat.

The diurnal rhythm of plasma corticosterone (B) in adult male rats was investigated 6, 12, 18, 24, 30 and 36 weeks after frontal isolation of the medial basal hypothalamus (MBH) and was compared to that of sham-operated controls. Taking blood samples by heart puncture, it was possible to follow the same groups of animals during the entire experimental period. In accordance with earlier reports, frontal deafferentation of the MBH abolished the diurnal rhythm of plasma B 6 weeks after surgery. No change was observed in this respect until the 30th postoperative week. At that time, however, the rhythm tended to return and was present by the 36th week in the operated animals.