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Recent evidence shows that thyrotrophin-releasing hormone (TRH) immunoreactivity in the rat anterior pituitary gland is accounted for by the TRH-like tripeptide prolineamide-glutamyl-prolineamide (pGlu-Glu-ProNH₂, <EEP-NH₂). The present study was undertaken to investigate further the regulation, localization and possible intrapituitary function of <EEP-NH₂. Anterior pituitary levels of <EEP-NH₂ were determined between days 5 and 35 of life, during the oestrous cycle and after treatment with the luteinizing hormone-releasing hormone (LHRH) antagonist Org 30276. Treatment of adult males with the LHRH antagonist either for 1 day (500 µg/100 g body weight) or for 5 days (50 µg/100 g body weight) reduced anterior pituitary <EEP-NH₂ levels by 25–30% (P<0·05 versus saline-treated controls). Anterior pituitary <EEP-NH₂ increased between days 5 and 35 of life. In females, these levels were 2- to 3-fold 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 levels in female rats decreased in parallel with a decrease in plasma FSH. Injections with the LHRH antagonist (500 µg/100 g body weight), starting on day 22 of life, led to reduced contents of <EEP-NH₂ in the anterior pituitary gland of female rats on days 26 and 30 (55 and 35% decrease respectively). Levels of <EEP-NH₂ in the anterior pituitary gland did not change significantly during the oestrous cycle. Fractionation 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 hormone (GH) release from anterior pituitary cells obtained from neonatal rats, but no consistent effects were seen on the in vitro release of luteinizing hormone (LH), FSH, prolactin (PRL) or thyroid-stimulating hormone (TSH) under basal or TRH/LHRH-stimulated conditions. Furthermore, <EEP-NH₂ did not affect the in vitro hormone release by anterior pituitary cells obtained from adult rats. In vivo, <EEP-NH₂ (0·3–1·0 µg intravenously) did not affect plasma PRL, TSH, LH, FSH and GH in adult male rats. We conclude that <EEP-NH₂ in the anterior pituitary gland is regulated by LHRH, is probably localized in gonadotrophs and may play a (paracrine) role in neonatal GH release.

Journal of Endocrinology (1995) 146, 293–300

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