Hyperthyroidism decreases thyrotropin-releasing hormone gene expression inthe caudal raphe nuclei and the parapyramidal regions in rats

Altered thyroid statuses are associated with autonomic disorders. Medullary thyrotropin-releasing hormone (TRH) and substance P (SP) regulate autonomi c nervous activity. The influences of thyroid statuses on TRH and SP gene e xpressions in the caudal raphe nuclei and the parapyramidal regions were st udied using quantitative in situ hybridization histochemistry. In male rats thyroidectomized (Tx) for 30 days, the serum T-4 levels decreased by 64% a nd the medullary pro-TRH mRNA signals (silver grains per neuron) significan tly increased by 32-45%. These changes were prevented by daily i.p. T-4 (2 mu g/100 g) injection in Tx rats. In sham operated/T-4 (20 mu g/100 g, dail y) injected rats, T-4 levels significantly increased by 88% and the silver g rains decreased by 38-40%. Medullary SP mRNA signals were not significant ly:changed by altering thyroid status. These results support the concept th at thyroid hormone regulates medullary TRH gene expression by negative feed back. (C) 1999 Elsevier Science Ireland Ltd. Ail rights reserved.