Ultrastructure and acetylcholinesterase content of neurones forming connections between the striatum and substantia nigra of rat.

Neurones in the striatum and substantia nigra (zona compacta) of the rat were divided into several classes according to differences in morphological appearance and ultrastructural localization of acetylcholinesterase (AChE). To determine which of the neurone types form connections between the substantia nigra and striatum, the ultrastructural localization of AChE was combined with retrograde axonal transport of horseradish peroxidase (HRP). The products of the histochemical reactions for HRP and AChE are localized in separate intracellular compartments and so identification of the efferent cell types was possible. In the striatum there are two morphological classes of AChE-containing neurone and two or three varieties of cell which do not stain for the enzyme. Only one neurone type projects to the substantia nigra and this is a cell with no AChE which makes up 96% of the total neurone population. In the substantia nigra there are two types of neurone with AChE: a cell with rather variable amounts of AChE which projects to the striatum, and another rarer type, possibly an interneurone.