Ultrastructural evidence for dendritic release of acetylcholinesterase in the rat substantia nigra

: Morphological evidence for dendritic secretion of acetylcholinesterase (AChE) in rat substantia nigra--a physiologically known phenomenon--was searched by means of a modified cytochemical method devised for fine localization of AChE activity at the electron microscopic level. DAB precipitate was observed in cluster of small vesicles in contact with the plasma membrane and in the extracellular space in the vicinity of the vesicles. Single coated or uncoated large vesicles filled with stained material were found in the cytoplasm of the dendrites at distance from or in contact with the plasma membrane. Immunoperoxidase staining with specific anti-serum against rat AChE gave similar localization of AChE. These results suggest that AChE is released from the dendrites of the nigral neurons by a process of vesicular exocytosis and captured by endocytosis. The relation of this process to a putative release from the smooth endoplasmic reticulum remains to be elucidated.