Neuropeptides in the pineal gland? A critical immunocytochemical study

In an attempt to localize components of the renin angiotensin-system in the pineal gland of rats, immunocytochemical studies using the PAP-technique were performed with antisera against angiotensin I, angiotensin II and angiotensinogen. The staining pattern thus obtained was not only the same for the three antisera, but was also identical to that shown for many other peptide-antisera in the literature. In those studies, the immunocytochemical staining had been ascribed to a distinct pineal cell population or to cell processes. However, by examining adjacent semithin and ultrathin sections by immunocytochemistry and electron microscopy, respectively, we could identify the extracellular perivascular compartment and its flocculent material as the site of staining. This unexpected localization and the observation of "immunoreactivity" of some preimmunesera in the same compartment as well as several additional findings and arguments are taken to suggest that likelihood of "pseudopositive" immunostaining, typical for the pineal gland.