Survey of Chemically Defined Cell Groups and Pathways

Abstract

Acetylcholine was the first neurotransmitter to be identified. Choline acetyltransferase (ChAT) is the enzyme which is responsible for the synthesis of ACh, whereas the enzyme acetylcholinesterase (AChE) degrades this substance (Fig. 1). In numerous older studies, among which those of Shute and Lewis (Shute and Lewis 1967; Lewis and Shute 1967) especially should be mentioned, it was attempted to study the anatomy of central cholinergic neurons by means of AChE histochemistry. However, it is known that although cholinergic neurons generally show a very high content of AChE, a high activity of this enzyme is in itself not a sufficient characteristic for identifying cholinergic neurons (Lehmann and Fibiger 1979; Butcher and Woolf 1982; Eckenstein and Sofroniew 1983). The successful purification of ChAT and the subsequent procurement of antibodies against this enzyme for use in immunochemistry have recently yielded a reliable means of visualizing cholinergic neurons in the central nervous system (cf. McGeer et al. 1984 a; Wainer et al. 1984; Rossier 1984). The following survey of the major central cholinergic neuron populations and their efferent projections (Fig. 2) is based primarily on the studies of Kimura et al. (1981), Fibiger (1982) and Mesulam et al. (1983a, b, 1984a).