Immunocytochemical distribution in rat brain of putative peptides derived from thyrotropin-releasing hormone prohormone.

Processing of the TRH prohormone (Pro-TRH), a protein of approximately 26,000 mol wt, could yield 5 copies of TRH, as well as extended forms of TRH and several other non-TRH peptides. To determine whether some of these peptides are formed and transported by axons in the rat brain, we used antiserum to synthetic peptides corresponding to portions of pro-TRH. These included the N-tyrosyl analogs [Tyr0]prepro-TRH-(25-50) (pYE27) and [Tyr1]prepro-TRH-(53-74) (pYT22) contained within the N-terminal flanking region of the prohormone, the N-tyrosyl analog [Tyr0]prepro-TRH-(165-186) (pYS23), expanding the fourth progenitor sequence of TRH in the midportion of the prohormone, and the synthetic peptide pAC12 corresponding to the first 12 amino acids of the C-terminal flanking region or prepro-TRH-(208-219). All antisera showed staining in neuronal perikarya and processes in all regions of the brain previously demonstrated to immunostain for TRH, including dense innervation of the external zone of the median eminence. In addition, these antisera immunostained regions of the brain not previously immunopositive for TRH. Not all regions reactive with antiserum to [Tyr0]prepro-TRH-(25-50) were also recognized by anti-pYT, -pYS, and -pAC. These studies confirm the presence of the deduced non-TRH sequences within the TRH precursor and their formation and transport in vivo in the central nervous system. The presence of immunoreactivity in regions of the brain that do not contain TRH and the variability of immunostaining of the different antisera in some of these regions suggest regional preferential processing of pro-TRH to other peptides that may be biologically active.