Participation of Serotonin in Thyrotropin Release. I. Evidence for the Action of Serotonin on Thyrotropin Releasing Hormone Release*

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Abstract
Injection of serotonin (5-HT) into the 3rd ventricle of the rat resulted in a rapid increase of serum TSH [thyrotropin]; a significant effect was observed 5 min after injection, whereas the maximal effect appeared 10 min after the injection of 1 .mu.g 5-HT. This stimulating effect of 5-HT was completely prevented by pretreatment with cyproheptadine, a blocker of 5-HT receptors, whereas fluphenazine, a dopamine receptor blocker, was unable to block it. Third ventricle injection of 5-HT in rats bearing anterior hypothalamic lesions (which did not affect the suprachiasmatic nucleus or the medio-basal hypothalamus) also induced an increase of serum TSH similar to that observed in normal rats despite the fact that these animals show a lower basal TSH. In vitro, the addition of 5-HT to an incubation medium containing 1 hemi-anterior pituitary did not modify medium TSH, whereas 5-HT addition induced an increase of medium TSH in the system containing 1 hemi-anterior pituitary and 2 hypothalami. 5-HT acts on TSH function probably through a stimulation of TRH [thyrotropin releasing hormone] release.