Circadian corticosterone rhythm did not develop in rats seven weeks after destruction with 5,7-dihydroxytryptamine of the serotoninergic nerve terminals in the suprachiasmatic nucleus at the age of 16 days.

In order to study the role of the serotoninergic innervation of the hypothalamic suprachiasmatic nucleus (SCN) in the development of the circadian rhythm of corticosterone 5,7-dihydroxytryptamine (5,7-DHT) selectively destroying the serotoninergic structures was injected into the cell group of 16-day-old male rats prior to the appearance of the corticosterone rhythm. Rats treated with 5,7-DHT did not show circadian fluctuations in plasma corticosterone concentrations 3, 5 and 7 weeks after the injection of the neurotoxin. Their hypothalamo-pituitary-adrenal system responded to ether stress. Immunocytochemistry revealed that only a very few serotonin immunoreactive elements were visible in the SCN of the 5,7-DHT-treated rats. The results indicate that serotoninergic innervation of the SCN is essential for the development of the corticosterone rhythm.