Discrepancy between Prolactin (PRL) Messenger Ribonucleic Acid and PRL Content in Rat Fetal Pituitary Cells: Possible Role of Dopamine E. L. Hooghe-Peters, A. Belayew, P. Herregodts, B. Velkeniers, G. Smets, J. A. Martial, and L. Vanhaelst Department of Pharmacology (E.L.H-P., B.V., L.V.) Pharmaceutical Institute (P.H.) Department of Experimental Pathology (G.S.) Medical School Vrije Universiteit Brussel B-1090 Brussels, Belgium Genetic Engineering Laboratory (A.B., J.A.M.) Universite de Liege B-4000 Sart-Tilman, Belgium Data are controversial concerning the time when PRL-synthesizing cells are detected for the first time in the rat pituitary. Using a very sensitive immuno- cytochemical technique, we could visualize only a few PRL cells before day 10 after birth. At that time, pituitary PRL was still 200 times less abundant than in the adult (on a tissue weight basis) whereas PRL mRNA per mg total RNA was only 80 times lower than in the adult. However, by in situ hybridization, we could demonstrate the presence of PRL mRNA in cells from fetal day 18 on. We have also followed the expression of GH gene in rat pituitary cells during development. In contrast to results obtained with PRL cells, quantitative analysis of cDNA probe hy- bridization to GH mRNA correlated well with meas- urements of immunostained cells. We found that PRL was released in the blood from fetal day 19 onwards. Thus, at that time PRL is synthesized and secreted but not stored. We therefore measured brain dopamine levels, and the data support the idea that the rise in dopamine levels after birth contrib- utes to PRL storage. We confirmed in vitro that newborn pituitary cells can store PRL when cultured in the presence of dopamine. (Molecular Endocri- nology 2: 1163-1168,1988) INTRODUCTION The rat anterior pituitary gland contains at least six distinct types of endocrine cells, expressing the follow- ing hormones: ACTH, TSH, PRL, GH, FSH, and LH (1). Attempts to trace the cell lineages and even the timing 0888-8809/88/1163-1168$02.00/0 Molecular Endocrinology Copyright © 1988 by The Endocrine Society of pituitary cell commitment is hampered by the com- plexity of a small organ made of a large variety of cell types. These cells divide and differentiate according to separate schedules and are under the control of various humoral (mainly neuroendocrine) and cellular (paracrine) factors. In order to overcome some of these problems, we have used the following strategy to monitor the ontogeny of PRL- and GH-producing cells. We have followed the expression of PRL and GH genes in rat pituitary cells during development. Dissociated cells were analyzed for their cytoplasmic mRNA content by in situ hybridization and for hormone storage by im- munoperoxidase cytochemistry. Quantitative analysis of cDNA probe hybridization to GH mRNA correlated well with measurements of immunostained cells. In contrast there was a discrepancy between PRL mRNA expression (high from fetal day 18) and PRL storage (low until 10 days postnatal). Very few PRL immunore- active cells could indeed be observed around birth. These data were confirmed by analysis of PRL mRNA and PRL extracted from adult vs. neonatal pituitaries. However, by assaying PRL in the plasma we could show that PRL was already released in the blood at fetal days 19-21; after birth, plasma PRL concentration dropped. One likely explanation was that PRL was already synthesized in the cell but could not yet be stored. It is suggested that dopamine (DA) is required for storage. We observed that brain DA levels raise after birth. In addition newborn pituitary cells can ac- cumulate PRL when cultured in the presence of DA. RESULTS In a first approach, the developmental expression of GH and PRL genes was studied in dissociated rat 1163 Downloaded from https://academic.oup.com/mend/article-abstract/2/12/1163/2713522 by guest on 24 May 2020