Effects of thyroid state on serotonin, 5-hydroxyindoleacetic acid and substance P contents in discrete brain nuclei of adult rats.

The effects of chemical thyroidectomy produced by propylthiouracil treatment or hyperthyroidism produced by daily injections of thyroxine on the content of serotonin, its metabolite 5-hydroxyindoleacetic acid and of substance P in discrete brain nuclei of adult rats have been studied. Brain nuclei were removed by punches of frozen brain slices. The serotonin and 5-hydroxyindoleacetic acid contents were measured by high performance liquid chromatography with electrochemical detection while substance P was assayed by radioimmunoassay. The serotonin level was significantly increased in 11 nuclei of the hypothyroid and in 12 nuclei of the hyperthyroid rats. The 5-hydroxyindoleacetic acid levels were higher in 16 nuclei of the hypothyroid and 7 nuclei of the hyperthyroid animals. The state of the thyroid gland in adult rats also affected the substance P system but less consistently than the serotonin pathway. Thus, the substance P content in the brain of propylthiouracil-treated animals increased significantly in 4 nuclei while it decreased in the septum lateralis and the striae terminalis. Following the chronic thyroxine injections, the substance P level was increased in the nucleus caudatus putamen and the tractus diagonalis and was decreased in the area ventralis tegmenti. We previously reported that neonatal thyroidectomy caused a general increase of the substance P and serotonin contents in many brain nuclei of young rats. It appears that the effects of chemical thyroidectomy on mature and immature rat brain are different for the substance P system, suggesting that thyroxine plays an important role in the maturation of substance P-containing neurons.