Cholinergic and Monoaminergic Systems of the Brain

Abstract

RECENT applications of histochemistry to the study of neurology, supplemented where necessary by appropriate surgical or pharmacological procedures, have led to the discovery of two major projection systems, both arising in part from the reticular formation of the brain stem. Neurones of one system, described and illustrated by us in the rat brain^1,2, contain acetylcholinesterase (AChE) throughout their length and are believed to be cholinergic, releasing acetylcholine as a transmitter substance from their terminals. Those of the other system, described in the rat by Dahlström and Fuxe^3,4, contain monoamines (MA)—either a catecholamine CA (noradrenaline NA or dopamine DA) or 5-hydroxytryptamine (5HT)—and are believed to be monoaminergic. The cholinergic neurones are evident in formalin-fixed frozen sections treated by the thiocholine method, and the monoaminergic neurones become fluorescent in freeze-dried material exposed to formaldehyde gas. Now that the findings of the Swedish workers have been published in full, it is possible to compare in detail the extent and distribution of the two systems, and to consider their inter-relationships.