Preprothyrotropin-releasing hormone178–199 affects tyrosine hydroxylase biosynthesis in hypothalamic neurons.

ProThyrotropin-releasing hormone (proTRH) is a prohormone widely distributed in many areas of the brain. After biosynthesis, proTRH is subjected to post-translational processing to generate TRH and seven non-TRH peptides. Among these non-TRH sequences, we found previously that preproTRH_178–199 could regulate the secretion of prolactin in suckled rats by their pups. Dopamine (DA), the main regulator of prolactin secretion, is produced in dopaminergic tyrosine hydroxylase (TH)-positive neurons in the hypothalamic arcuate nucleus (ARC). In this study we investigated whether prolactin release during the estrous sexual cycle is regulated by prepro TRH_178–199 through its effecton DA neurons of the ARC. We observed that biotinylated prepro TRH_178–199 bound to neurons in the ARC; this was higher during proestrus than during diestrus. Binding of preproTRH_178–199 to DA neurons was seen only during proestrus in the ARC. Using primary neuronal hypothalamic cultures we found that preproTRH_178–199peptide decreased TH levels in a dose-responsive manner, whereas intra-ARC administration of preproTRH_178–199 induced a 20-fold increase in plasma prolactin levels. Together, these results suggest a potential role for preproTRH_178–199 in regulating dopaminergic neurons involved in the inhibition of pituitary prolactin release. [ABSTRACT FROM AUTHOR]