Processing of Thyrotropin-Releasing Hormone (TRH) Prohormone in the Rat Olfactory Bulb Generates Novel TRH-Related Peptides*

Abstract

Based on the deduced amino acid sequence of rat TRH prohormone (pro-TRH), proteolytic processing of this polyprotein precursor is expected to produce, beside TRH, several other novel peptides. These peptides should correspond to connecting segments flanking the repeated TRH progenitor sequence and to various C- and/or N-terminally extended forms of TRH. The profile of the endogenous products of the TRH system was studied in rat brain using multiple RIAs coupled to molecular sieve filtration and HPLC separations. In extracts from the rat hypothalamus, TRH and two pro-TRH-connecting peptides, prepro-TRH-(160-169) and prepro-TRH-(178-199) were detected in molar ratios corresponding to those expected for a nearly complete processing of the prohormone molecule. In the olfactory bulb, pro-TRH is processed differently, since peptides containing TRH at their N-termini, [pGlu172] prepro-TRH-(172-199) and [pGlu154]prepro-TRH-(154-169), were found to be major end products along with prepro-TRH-(160-169) and prepro-TRH-(178-199). The dissimilarity in tissue content suggests that differential processing of TRH precursor by various enzymatic pathways may act as a regulating mechanism for TRH and TRH-related activities. The cellular localization of C-terminally extended forms of TRH in rat olfactory bulb was examined by the indirect immunoperoxidase method, using antisera directed against prepro-TRH-(160-169) and pre-pro-TRH-(178-199). Cell bodies and nerve fibers were detected in the glomerular and external plexiform layers of the main olfactory bulb. The presence of extended forms of TRH in interneurons and middle tufted cells of the main olfactory bulb suggests that in light of the recent biological properties described for prepro-TRH-(160-169), these peptides may act as neuromodulators for olfactory epithelium inputs or neurotransmitters within more rostrally located olfactory areas in the forebrain.

Electroconvulsive Seizures Increase Levels of PS4, the TRH-Enhancing Peptide [Prepro-TRH(160–169)], in Rat Brain

“…has been reported to pro mote the expression of TSH (i-subunit mRNA in rat thyrotropes [ l], to enhance rat pituitary synthesis and release of prolactin [ l], and to enhance the in vitro release of thy roid-stimulating hormone from rat anterior pituitaries in response to TRH (p-Glu-His-Pro-NH;;), but lacks intrin sic thyroid-stimulating hormone releasing activity [2,3], Infusion of PS4 into the raphe nucleus of the medulla also increases TRH-induced gastric acid secretion of rats [4], A specific receptor for PS4 has been identified in the plas ma membrane of rat anterior pituitaries [5, 6], We have previously reported that electroconvulsive seizures (ECS) produce a sustained increase in TRH and TRH-Gly in limbic regions of the rat brain which corre lates with an antidepressant effect as measured in the Porsolt forced swim test [7). mRNA levels for enzymes responsible for proteolytically cleaving prepro-TRH, and other neuropeptides, are also increased by ECS [8].…”

Distribution, Cellular Localization, and Ontogeny of Preprothyrotropin-Releasing Hormone-(160–169) (Ps4)-Binding Sites in the Rat Pituitary1

“…Most of these studies have been focused on the fourth connecting peptide, termed Ps4, a decapeptide (Ser-Phe-Pro-Trp-Met-Glu-SerAsp-Val-Thr) linking the third and fourth copies of the TRH progenitor. It was first demonstrated that posttranslational processing of pro-TRH generates authentic Ps4 in the olfactory lobe, hypothalamus, and spinal cord (2) and in the pancreas (3). Immunocytochemical studies have subsequently revealed the existence of a dense accumulation of Ps4-containing nerve terminals in the external zone of the median eminence (4).…”

Thyrotropin-releasing Hormone Immunoreactivity in Rat Adrenal Tissue Is Localized in Mast Cells

“…Proteolytic processing of the rat precursor is expected to produce five copies of TRH along with several connecting peptides and various C-and/or N-terminally extended forms of TRH. A variety of data have provided evidence for the coexistence of partial and complete processing of pro-TRH in various regions of the rat brain (Cockle and Smyth 1987;Wu et al 1987;Bulant et al 1988). Like other neural peptides, TRH occurs outside the CNS in neuroendocrine cells.…”