Inhibition of prolactin secretion from the male rat anterior pituitary by cryptic sequences of prothyrotropin releasing hormone, ProTRH_178-199 and ProTRH_186-199

Previous studies have shown that intronic peptide sequences in the prohormone for thyrotropin-releasing hormone (TRH) have physiological actions on pituitary hormone secretion. The aim of this investigation was to examine the effect of the cryptic peptides, prothyrotropin-releasing hormone_178-199 (ProTRH_178-199) and ProTRH_186-199, on prolactin (PRL) release from the anterior pituitary. Perifusion studies were performed with anterior pituitaries obtained from individual adult male Sprague-Dawley rats at 70-90 d of age. Perifusate was collected in 5-min fractions for 25 min prior to peptide administration and for 60 min afterward. Pituitaries were perifused with a single 5 min pulse of either 2, 10, or 40 nM concentrations (peak pulse) of each peptide or the vehicle. Sixty minutes after peptide administration, a 200 mM pulse of potassium chloride was delivered to check tissue viability. Prolactin was measured in the perifusate by radioimmunoassay. Results showed that both peptides induced a significant long-term suppression of prolactin secretion that was still evident at 60 min after peptide exposure. ProTRH_186-199 was similar to ProTRH_178-199 in suppressing prolactin release at the 2 and 40 nM dose, suggesting that the amino acid sequence necessary for prolactin inhibition is contained within the smaller peptide fragment. These data indicate that a cryptic sequence within the proTRH peptide can have biological activity at the level of the anterior pituitary gland in regulating prolactin secretion.