Ultrastructural investigation of ACTH immunoreactivity in arcuate and supraoptic nuclei of the rat.

Fine structural localization of an ACTH-like substance was obtained in neurons of the rat arcuate nucleus using immuno-electron microscopy, whereas it could not be confirmed that ACTH-containing cell bodies are present in the supraoptic nucleus. The immunoreactive cells of the arcuate nucleus appeared to be more numerous than the unreactive neurons. Immunostaining was carried out before embedding in resin. Empty vesicles of irregular shape were found in dendrites of immunoreactive arcuate neurons, but their significance and nature remain enigmatic. The reaction product was distributed uniformly throughout the cytoplasm of the ACTH-positive cells, except that the mitochondria, rough endoplasmic reticulum and Golgi vesicles and cisternae were devoid of PAP molecules. This distribution differed from the localization reported in ACTH-secreting cells of the rat anterior pituitary, where the reaction product was found in the rough endoplasmic reticulum and Golgi complex as well as in secretory granules.