Radioautographic study of binding and internalization of corticotropin-releasing factor by rat anterior pituitary corticotrophs.

In order to identify the anterior pituitary cell type(s) containing corticotropin-releasing factor (CRF) receptors and to study the internalization processes of this peptide by the target cells, radioautography was performed on rat anterior pituitaries removed at specific intervals (2-60 min) after intracarotid injection of [125I]iodo-CRF into intact and adrenalectomized female rats. In intact animals, all corticotrophs were labeled, whereas in the adrenalectomized animals about 80% of the hypertrophied corticotrophs (adrenalectomy cells) were. In control animals injected with both iodinated CRF and an excess of unlabeled peptide, no specific reaction could be detected. The time-course study in intact animals showed that 2 min after injection most silver grains were found over or within 160 nm of the plasma membrane. At the 5-min time intervals, grains were observed both over the plasma membrane and within the cytoplasm, associated with lysosomes, and the Golgi apparatus. Fifteen minutes after injection, grains were mostly found over lysosomes and the Golgi apparatus, whereas at the longest time intervals (30 and 60 min) almost no labeling could be detected. The results obtained in this study indicate that in the anterior pituitary CRF receptors are restricted to corticotrophs (as identified by electron microscopy) and that, after binding to the plasma membrane, CRF is rapidly internalized to Golgi elements and lysosomes.