The Cryptic Peptides, Prepro-Thyrotropin Releasing Hormone 186-199 and 194-199, Suppress Anterior Pituitary Prolactin Secretion in vivo and in vitro

Abstract

Prepro-thyrotropin releasing hormone (ppTRH)-176-199 is one of several peptide fragments cleaved during TRH synthesis and has been implicated as a regulator of neuroendocrine function. ppTRH 176-199 has been shown to acutely inhibit the stress-induced rise in ACTH, corticosterone (CORT), and prolactin (PRL) in the rat. The receptor for ppTRH 176-199 currently remains unknown. In this study we sought to characterize the active domain of ppTRH 176-199 and, using in vivo and in vitro approaches, determine its role in regulating anterior pituitary secretion of PRL. The 186-199, 194-199, and 186-191 amino acid fragments of ppTRH were administered I.P. to adult male Sprague-Dawley rats 15 min. prior to a 20 min restraint stress to determine the peptide’s active moiety in regulating prolactin secretion. Animals were euthanized and plasma was saved for assay of circulating PRL using enzyme immunoassay (EIA). ppTRH 186-199 significantly attenuated the stress-induced rise in prolactin in male rats in a dose-responsive fashion. This effect was mimicked by ppTRH 194-199 but not by ppTRH 186-191. At the highest dose (10 mg/kg BW), ppTRH 194-199 also reduced the stress-induced rise in plasma CORT. Additionally, in vitro studies were performed using the rat growth hormone (GH)/PRL –secreting MMQ cell line. MMQ cells were treated with ppTRH 186-199 and media was assayed for PRL levels. Cells were harvested and examined for changes in PRL mRNA. Within 30 minutes following treatment of estradiol-stimulated MMQ cells with ppTRH 186-199 there was a decrease in media levels of PRL compared to vehicle. Furthermore, in MMQ cells that were primed with 10nM estradiol for 48 hours there was an increase in media PRL levels, which was reduced following ppTRH 186-199 treatment. After 4 hrs of treatment, the inhibitory effect of ppTRH 186-199 on PRL secretion from MMQ cells was only seen on estradiol-stimulated cells. There were no effects of ppTRH 186-199 when examined after 24 hrs of treatment. There were no effects of ppTRH 186-199 or 194-199 of PRL mRNA levels. These data suggest that the carboxy terminal fragment of preproTRH 178-199 contains all the activity of this ppTRH cryptic peptide for regulation of PRL and corticosterone secretion. This suggests a potential moiety responsible for interaction with the peptide’s receptor. The inhibitory effect of ppTRH 186-199 and 194-199 on media PRL levels and not on mRNA synthesis implicates it as an effector of hormone secretion rather than protein synthesis. The short-lived duration of its effects supports a role as 6 an acute effector of the PRL system. The target receptor of the ppTRH 178-199 fragment remains uncertain. However the use of ppTRH 194-199 as a peptide bait may prove useful in identifying the receptor.