Cytochemical localization of monoamine oxidase in the pig pineal gland

Monoamine oxidase plays an important role in the regulation of monoamine levels in the pineal gland by oxidative deamination of serotonin to 5-hydroxyindole acetic acid. The ultrastructural sites of this process are still not well known. In the present study the pig pineal glands were examined by using two cytochemical methods for demonstration of MAO activity at the ultrastructural level. The use of selective inhibitors of MAO-A (clorgyline and amiflamine) and MAO-B (deprenyl) showed that MAO-A was localized predominantly in noradrenergic nerve terminals as well as in the cell membrane of endothelial cells. MAO-B activity was localized in pinealocytes. However, in the outer membranes of cytoplasmic dense bodies, which are characteristic cytoplasmic structures of pig pinealocyte, we noticed the activity of both MAO-A and MAO-B enzymes. The MAO-A was localized exclusively in dense bodies within bulbous ending of pinealocyte processes present in the perivascular space. Our results show that in pig pineal gland there are two forms of MAO localized in different compartments of the pineal tissue. The most important result of our study is the presence of MAO in cytoplasmic dense bodies, which points to these bodies as to the sites of oxidative deamination and confirms their participation in the secretory process in pig pineal gland.