Effect of intracerebral serotonin administration on pituitary-thyroid function

Thyroid function was investigated following intrahypothalamic implantation of cannulae containing 80 mug serotonin-creatine sulphate or after intraventricular injection of 100 mug of the same substance. Control animals received either needle implantations or intraventricular injections of creatine-sulphate. Thyroid/serum 131-I ratio, activity of the thyroid gland as evaluated histologically and thyrotrophic hormone (THS) content in the pituitary were significantly decreased in serotonin-treated animals. Thyrotrophic hormone releasing hormone (TRH) concentration of the hypothalamus was also significantly lowered in serotonin-treated animals when compared to intact controls. Creatine-sulphate injected rats showed a slight decrease of TSH content of the pituitary and of TRH concentration of the hypothalamus which is probably due to a stress effect caused by the intracranial intervention and/or by the intraventricular administration of a relatively large molecule, such as creatine sulphate. It is concluded that serotonin decreases the functional activity of the hypothalamo-pituitary-thyroid system by inhibiting the TRH-secretion of the hypothalamus.