The localization of acetylcholinesterase in the locus coeruleus of the normal rat and after 6-hydroxydopamine treatment.

The locus coerulus is a densely packed group of neurons in the floor of the fourth ventricle, and is the largest aggregate of noradrenaline-containing cells in the mammalian brain. The distribution within the locus of acetylcholinesterase (AChE), which is present in high concentration, has been studied at light and electron microscope level, both in normal rats and in ones treated with 6-hydroxydopamine. Neuronal enzyme activity is entirely intracellular and mainly concentrated in stacks of ER which occupy much of the cell cytoplasm. There are no indications of a cholinergic input. After 6-hydroxydopamine treatment extensive cell death occurs and AChE activity virtually disappears. A majority of the many blood vessels in the locus also stain strongly for AChE, unlike those present in most other areas of the rat brain. The locus coeruleus therefore represents an area of the rat brain with a high content of AChE, but no evidence of a cholinergic mechanism. Some possible explanations for this anomalous presence of AChE are briefly discussed.