THE HYPOPHYSEAL PARS TUBERALIS IS ENRICHED WITH DISTINCT PHOSPHOTYROSINE-CONTAINING PROTEINS NOT DETECTED IN OTHER AREAS OF THE BRAIN AND PITUITARY

The regulation of cell activity, growth and metabolism by a number of growth factor receptors and proto-oncogene products involves tyrosine kinase activity resulting in autophosphorylation of the receptors and production of phosphorylated tyrosine-containing protein substrates. T he identification and precise localization of phosphotyrosine (PY)-con taining proteins are first steps in elucidating the functional role of tyrosine kinases in the modulation of the central nervous system and related areas. In the present report, we describe PY-containing protei ns in the median eminence and adjacent pars tuberalis of the rat adeno hypophysis by immunocytochemistry using light and electron microscopy, and by Western blotting analysis. PY-immunoreactivity was found to be most intense throughout the cytoplasm of a population of epithelial p ars tuberalis cells. Polyacrylamide gel electrophoresis and Western bl otting of tissue extracts from various brain and pituitary regions dem onstrated a general pattern of 4 major bands of PY-proteins, with an a dditional dense band representing a 44 kDa protein that was highly pho sphorylated on tyrosines and that was exclusively found in the pars tu beralis. Additional investigation for the presence of insulin receptor s, a tyrosine kinase previously correlated with the distribution of PY -proteins, demonstrated a receptor localization in axons and nerve ter minals in the external and internal zone of the median eminence. Howev er, the large amount of different PY-proteins present in the secretory cell population of the pars tuberalis could not be attributed to the insulin receptor. Our findings demonstrate that there is a large amoun t of cell-specific tyrosine kinase activity in the median eminence and contacting the pars tuberalis; these may play a significant role for transduction of biological signals or metabolic regulation in the neur oendocrine region.