The role of central serotoninergic and noradrenergic neurons in the regulation of nyctohemeral rhythm of plasma thyrotropin.

A possible participation of central serotoninergic or noradrenergic neurons on nyctohemeral periodicity of plasma TSH concentration was studied in the rat. Control rats exhibited normal nyctohemeral fluctuations of plasma TSH characterized by high AM (09:30) and low PM (18:30) values (397 +/- 45 vs. 207 +/- 28 ng/ml, p < 0.01). The daily ip administration of p-chlorophenylalanine (p-CPA, 100 mg/kg body weight) for 7 days produced significant decreases in plasma TSH concentration and hypothalamic TRH content in the AM. However, these parameters in the PM were unaffected by p-CPA administration. An inhibition of serotoninergic activity thus abolished nyctohemeral periodicity in plasma TSH by lowering only AM values (AM:253 +/- 32 vs. PM:236 +/- 30 ng/ml, p > 0.1). On the other hand, intraventricular administration of 6-hydroxydopamine (6-OHDA, 250 micrograms/rat), a drug able to deplete noradrenaline and possibly dopamine, had no appreciable effect on AM-PM fluctuations of either plasma TSH or hypothalamic TRH content. The rate of elevation of plasma TSH after thyroidectomy was not significantly different in p-CPA or 6-OHDA treated rats compared to the respective controls, suggesting that there is no significant contribution of these biogenic amines to feedback control of TSH secretion by thyroid hormones. The present study suggests that the nyctohemeral periodicity of pituitary TSH secretion can be produced by cyclic activation of hypothalamus brought about by the increased input of serotoninergic neurons in response to the periodicity of certain environmental cues and that central noradrenergic or dopaminergic neurons are not involved in this system.