Promiscuous hormone mRNA expression in the pituitary remains poorly understood. We examined by means of RT-PCR and immunostaining whether GSU could be coexpressed with POMC in vivo and under pressure of CRH in vitro. Cells coexpressing GSU and POMC mRNA amounted to 2.6% of the cells in ex vivo rat pituitary at birth (P1), fell to much lower level at P14 and were undetectable in adulthood. In cultured pituitary aggregates of P14 rats, GSU/POMC cells remained scarce but represented up to 6.6% after chronic treatment with CRH but not leukemia inhibitory factor. CRH was less effective in aggregates from P1 and adult rats. The total GSU population ex vivo at P1 was 2 times smaller than at P14, but in culture it expanded 2.5 times, concommittantly with a reciprocal change in POMC cell abundance. Tpit transcripts were detected in POMC-only and GSU/POMC cells but not in GSU-only cells. Cells coexpressing GSU and POMC mRNA were relatively abundant in P14 chicken pituitary and aggregate cultures, but occurrence was not affected by CRH. Immunostaining showed GSU and POMC colocalisation in sporadic cells in intact rat pituitary and CRH-treated cultures at P1 but not at P14 and adult age.

The data demonstrate the occurrence of cells coexpressing GSU and POMC in rat and chicken pituitary. The developmental dynamics of this cell population and its response to CRH in vitro in the rat, suggests a relationship of these cells with the embryonic branching of the POMC and GSU cell lineages and their mutually opposite developmental course during early postnatal life.