Participation of serotonin in thyrotropin release. II. Evidence for the action of serotonin on the phasic release of thyrotropin.

During a 12-h light 12-h dark schedule (lights off at 1900 h), male Sprague-Dawley rats show a circadian rhythm of plasma TSH with a zenith near midday. The participation of serotonin (5HT) in the phasic release of TSH was studied using both pharmacological and surgical-stereotaxical approaches. Animals treated with parachlorophenylalanine methyl ester (pCPA), an inhibitor of 5HT synthesis (one or two injections of 250 mg/kg each) showed a reduction or a disappearance of the diurnal peak of TSH, respectively. Additional treatment by 5-hydroxytryptophan, a precursor of 5HT, completely, restored the diurnal TSH peak. Treatment with 5,6-dihydroxytryptamine creatine sulfate, a neurotoxin which selectively destroys 5HT terminals, also induced alterations of the diurnal peak of TSH. There were no major modifications observed in the low nocturnal levels of TSH in rats treated with pCPA, 5-hydroxytryptophan, or 5,6-dihydroxytryptamine. The major serotoninergic innervation of the hypothalamus originates from the raphe dorsalis or centralis; destruction of these two nuclei caused a quasiabolition of the diurnal TSH peak (only a low amplitude TSH circadian rhythm persisted). Hypothalamic 5HT content was measured in the majority of these experiments; the greatest depletions (near 90%) were observed after two injections of pCPA or in rats bearing raphe lesions. We conclude that the diurnal peak of TSH, observed during the physiological circadian rhythm, is serotoninergic dependent.