Further studies on circadian hormone rhythms after local pharmacological destruction of the serotoninergic innervation of the rat suprachiasmatic region before the onset of the corticosterone rhythm.

In previous studies we observed that local pharmacological destruction of the serotoninergic innervation of the hypothalamic suprachiasmatic nucleus before the onset of the circadian plasma corticosterone rhythm interferes with the appearance of the corticosterone rhythm up to the age of two months. In the present investigations we studied other hormone rhythms in such rats and tested animals for corticosterone rhythm when they were older than two months. We found normal circadian fluctuations in plasma testosterone and prolactin levels, variations in growth hormone concentrations, but no changes in plasma corticosterone levels of 63-day-old rats receiving 5,7-dihydroxytryptamine, a neurotoxin selectively destroying the serotoninergic structures, into the suprachiasmatic nucleus at the age of 16 days. Rats did show circadian variations in plasma corticosterone concentrations when tested 3 months after treatment with the neurotoxin. In these latter animals, a significant amount of serotonin-immunoreactive fibers and terminals were evident in the suprachiasmatic nucleus. Only a very few of such elements were seen in rats with a shorter postoperative survival. Our data support the view that serotoninergic innervation of the suprachiasmatic nucleus is essential for the onset of the circadian fluctuations of plasma corticosterone concentrations.