SIMULTANEOUS LOCALIZATION OF PIT-1 PROTEIN AND GONADOTROPINS ON THE SAME CELL-TYPE IN THE ANTERIOR-PITUITARY GLANDS OF THE RAT

Pit-1 is a prototypic member of the POU transcription factor family an d plays a critical role in pituitary-specific action of growth hormone (GH), prolactin (PRL), and thyroid-stimulating hormone (TSH) beta-sub unit genes. The purpose of the present study was to elucidate the chan ges in the expression of the Pit-1 product in the pituitary of pregnan t rats employing an improved double-immunohistochemical method. The po sitive cells showed nuclear immunoreactivity and Pit-1 protein was fre quently observed in the nuclei of many cells which were also immunopos itive for GH, PRL, and beta TSH. Unexpectedly, a significant number of pituitary cells containing both Pit-1 and gonadotropins were also obs erved. These cells were usually distributed near blood vessels that su pply the pituitary. While a prominent increase in the percentage of Pi t-1/PRL, Pit-1/beta-luteinizing hormone and Pit-1/beta-follicle-stimul ating hormone immunoreactive cells was observed in pregnant rats, the percentage of Pit-1/GH immunoreactive cells was strongly decreased. In contrast, no significant differences in the percentage of Pit-1/beta TSH doubly immunolabeled cells were noticed. Our findings strongly sup port the hypothesis that PRL could coexist in the Pit-1 immunopositive gonadotropes. Although Pit-1 protein was not detected in the nuclei o f corticotropes, the existence of these cells in the rat pituitary can not be excluded.