ONTOGENY OF PITUITARY TRANSCRIPTION FACTOR-I (PIT-1), GROWTH-HORMONE (GH) AND PROLACTIN (PRL) MESSENGER-RNA LEVELS IN MALE AND FEMALE RATS AND THE DIFFERENTIAL EXPRESSION OF PIT-1 IN LACTOTROPHS AND SOMATOTROPHS

Pituitary transcription factor-1 (Pit-1 or GHF-1), a transcription fac tor specific to the anterior pituitary, is involved in the expression and regulation of the growth hormone (GH) and prolactin (PRL) genes. P ost-pubertally, the expression of both GH and PRL becomes sexually dim orphic with males having higher GH levels and females higher PRL level s; however, little is known about the postnatal regulation of their co mmon transcription factor. Furthermore, whether the Pit-1 gene is diff erentially expressed in somatotrophs and lactotrophs remains to be elu cidated. In this study, we used in situ hybridization histochemistry t o examine Pit-1, GH and PRL mRNA levels in the anterior pituitaries of male and female rats throughout development (0, 5, 10, 20, 30, 40 and 60 days of age) to determine when GH and PRL production becomes sexua lly dimorphic and if this is accompanied by a dimorphism in Pit-1 gene expression. In addition, the level of Pit-1 mRNA was determined separ ately in both GH mRNA and PRL mRNA containing cells during the various developmental stages. We found that in both males and females the mRN A levels of Pit-1, GH and PRL remain relatively unchanged until around the time of pubertal onset (30-40 days) when there is a significant i ncrease in all three mRNA species, which is followed by a decrease to adult levels. Also around the time of puberty, both GH and PRL mRNA le vels become sexually dimorphic, with males having higher levels of GH mRNA and females higher PRL mRNA levels. In contrast, at no time durin g development were overall Pit-1 mRNA levels found to differ between t he sexes. However, when Pit-1 mRNA content was measured separately in specific cell types, significant differences between the sexes became evident. Throughout development Pit-1 mRNA levels are higher in lactot rophs of females than in those of males, whereas in somatotrophs males have higher Pit-1 mRNA levels than females. Furthermore, within a sex there is differential expression of Pit-1 in the two cell types with females having significantly higher levels of Pit-1 in lactotrophs tha n in somatotrophs and males having higher levels in somatotrophs than in lactotrophs. These data support the hypothesis that a sexual dimorp hism exists in the expression of the pituitary specific transcription factor Pit-1; however, this dimorphism is not manifest as a difference in overall mRNA levels, but in the differential expression of this ge ne in lactotrophs and somatotrophs.