FURTHER-STUDIES ON THE REGULATION, LOCALIZATION AND FUNCTION OF THE TRH-LIKE PEPTIDE PYROGLUTAMYL-GLUTAMYL-PROLINEAMIDE IN THE RAT ANTERIOR-PITUITARY GLAND

Recent evidence shows that thyrotrophin-releasing hormone (TRH) immuno reactivity in the rat anterior pituitary gland is accounted for by the TRH-like tripeptide pyroglutamyl-glutamyl-prolineamide (pGlu-Glu-ProN H(2), <EEP-NH2). The present study was undertaken to investigate furth er the regulation, localization and possible intrapituitary function o f <EEP-NH2. Anterior pituitary levels of <EEP-NH2 were determined betw een days 5 and 35 of Life, during the oestrous cycle and after treatme nt with the luteinizing hormone-releasing hormone (LHRH) antagonist Or g 30276. Treatment of adult males with the LHRH antagonist either for 1 day (500 mu g/ 100 g body weight) or for 5 days (50 mu g/100 g body weight) reduced anterior pituitary <EEP-NH2 levels by 25-30% (P<0.05 v ersus saline-treated controls). Anterior pituitary <EEP-NH2 increased between days 5 and 35 of life. In females, these levels were 2- to 3-f old higher (P<0.05) than in males between days 15 and 25 after birth; these changes corresponded with the higher plasma follicle-stimulating hormone (FSH) levels in the female rats. After day 25, <EEP-NH2 level s in female rats decreased in parallel with a decrease in plasma FSH. Injections with the LHRH antagonist (500 mu g/100 g body weight), star ting on day 22 of life, led to reduced contents of <EEP-NH2 in the ant erior pituitary gland of female rats on days 26 and 30 (55 and 35% dec rease respectively). Levels of <EEP-NH2 in the anterior pituitary glan d did not change significantly during the oestrous cycle. Fractionatio n of anterior pituitary cells by unit gravity sedimentation was found to be compatible with the localization of <EEP-NH2 in gonadotrophs. In vitro, <EEP-NH2 dose-dependently inhibited TRH-stimulated growth horm one (GH) release from anterior pituitary cells obtained from neonatal rats, but no consistent effects were seen on the in vitro release of l uteinizing hormone (LH), FSH, prolactin (PRL) or thyroid-stimulating h ormone (TSH) under basal or TRH/LHRH-stimulated conditions. Furthermor e, <EEP-NH2 did not affect the in vitro hormone release by anterior pi tuitary cells obtained from adult rats. In vivo, <EEP-NH2 (0.3-1.0 mu g intravenously) did not affect plasma PRL, TSH, LH, FSH and GH in adu lt male rats. We conclude that <EEP-NH2 in the anterior pituitary glan d is regulated by LHRH, is probably localized in gonadotrophs and may play a (paracrine) role in neonatal GH release.