Thyroid hormone modulation of thyrotrophin-releasing hormone (TRH) and TRH-Gly levels in the male rat reproductive system.

Thyrotrophin-releasing hormone (TRH) occurs in high concentrations in the rat ventral prostate and its concentrations is regulated in a positive dose-response manner by testosterone in castrated rats. alpha-Amidation of the tetrapeptide precursor, TRH-Gly, is a rate-limiting step in TRH biosynthesis. To investigate further the hormonal regulation of TRH biosynthesis in prostatic tissue, Sprague-Dawley rats of approximately 250 g were injected s.c. with either physiological saline or 3 mg propylthiouracil (PTU) daily for 5 days. The reproductive tissues were boiled in acetic acid (l mol/l), dried and extracted with methanol. The methanol extracts were measured for TRH immunoreactivity (TRH-IR) and TRH-Gly-IR by radioimmunoassay. Hypothyroidism induced by PTU significantly increased TRH-IR and TRH-Gly-IR levels in prostate and testis and reduced these levels in epididymis but did not affect the serum concentrations of testosterone compared with those of controls. Corresponding changes in TRH and TRH-Gly in the rat prostate were established by high-pressure liquid chromatography. To control for possible pharmacological effects of PTU on TRH biosynthesis, additional experiments were carried out on castrated rats receiving testosterone replacement and treatment with PTU plus methimazole. Treatment with thyroxine (T4) significantly reduced the increase in prostatic TRH levels due to hypothyroidism, despite the drug-induced blockade of the conversion of T4 to tri-iodothyronine. These effects parallel similar observations made in rat spinal cord and pancreas. This study demonstrates that in the male rat reproductive system the levels of TRH and its immediate biosynthetic precursor, TRH-Gly, are regulated by thyroid hormones.