IN-VITRO EVIDENCE THAT LHRH STIMULATES THE RECRUITMENT OF PROLACTIN MESSENGER-RNA-EXPRESSING CELLS DURING THE POSTNATAL-PERIOD IN THE RAT

We have studied the effect of LHRH and neuropeptide Y (NPY) on prolact in (PRL) mRNA levels in pituitary reaggregate cell cultures from 14-da y-old female rats, by means of in situ hybridization and Northern blot analysis. As estimated by computer-image analysis, addition of LHRH o n day 5 in culture for 40 h resulted in a 37% increase in the total cy toplasmic areas of cells containing PRL mRNA, visualized using a digox igenin-labelled PRL cRNA. The size of individual PRL-expressing cells was not influenced, nor was the content of PRL mRNA per cell. A simila r effect of LHRH was found by dot blot hybridization of extracted RNA, PRL mRNA levels were not affected by NPY. LHRH induced a 29% increase in the number of PRL mRNA-expressing cells processing through the S p hase of the cell cycle, visualized by the incorporation of [H-3]thymid ine ([H-3]T) into DNA over 16 h. The fraction of [H-3]T-labelled cells was 10-12% of the total cell population. NPY did not influence the nu mber of [H-3]T-positive cells expressing PRL mRNA, but completely bloc ked the effect of LHRH on the latter population. The present data sugg est that LHRH, probably via a paracrine action of gonadotrophs, stimul ates the recruitment of new lactotrophs, an action which is negatively modulated by NPY. Since the magnitude of this effect was the same in the total pituitary cell population as in cells processing through the S phase of the cell cycle and presumably mitosis, recruitment of lact otrophs seems to be based on differentiation of progenitor or immature cells into PRL-expressing cells, rather than on a mitogenic action on pre-existing lactotrophs alone.