Light- and electron-microscope differentiation of axons containing vasopressin and oxytocin in the neurohypophyseal lobe of the rat.

In neural lobes of Sprague-Dawley rats fixed with glutaraldehyde but without osmication, toluidine blue staining of semi-thin sections revealed two clear axon types: type I, highly stained, and type II, slightly stained. After uranyl acetate and lead citrate impregnation of adjacent ultrathin sections, electron microscopy showed that type I axons contained electron-dense granules whereas type II axons mostly contained pale granules. When adjacent semi-thin sections were stained with toluidine blue or treated for immunocytochemistry, type I axons were found to react with vasopressin antisera, and type II axons with oxytocin antisera. In neural lobes of Brattleboro rats, only type II axons were observed and contained pale granules. Both in light and electron microscopy, glutaraldehyde fixation without osmication, therefore provided a simple and reliable approach to distinguish between neurohypophyseal axons containing vasopressin and oxytocin, respectively.