Acetylcholine release from the central nervous system: a 50-year retrospective.

Some 50 years have elapsed since Elliot et al. and MacIntosh & Oborin first reported a release of acetylcholine (ACh) from canine and feline cerebral cortices, respectively. In this review, subsequent developments in the field during the succeeding five decades are explored. The arrangement of material in the review is outlined in this abstract, concluding with some suggestions as to its potential significance. A number of technical advances during this period have contributed to a greater understanding of the role that ACh may play in the central nervous system. These include the relatively recent evolution of the microdialysis and transverse dialysis techniques that enabled investigators to explore ACh release in deep regions of the brain. Future studies will likely be refined with the use of microelectrode biosensors, which should allow real-time measurements of ACh concentrations at the synaptic level. Controversies arising from the use of cholinesterase inhibitors and muscarinic receptor antagonists to enhance release are being resolved as a result of a better understanding of the presynaptic actions of these agents. Future studies will also benefit from the recent development of clostridial and other neurotoxins to reduce ACh release in areas of the brain. The likelihood that ACh may act as a cotransmitter at synapses in conjunction with glutamic acid, nitric oxide, and adenosine triphosphate is also explored. Attention is focused on the elucidation of choline acetyl-transferase (ChAT)-containing pathways in the central nervous system using techniques such as immunohistochemistry, in situ hybridization, histochemistry of ChAT mRNA, acetylcholinesterase histochemistry, and the distribution of the vesicular ACh transporter. Such studies have defined several major groupings of cholinergic neurons in the brain, which provide ascending or descending projections to higher and lower central structures. A major section of the review is devoted to actual studies on ACh release in the brain and spinal cord. This presentation is in two sections. The text details some of the material that has been obtained in experiments over the past 50 years. In five Tables, the results obtained in the majority of release studies to date are summarized. Although the data obtained to date clearly support the hypothesis that ACh is involved in electroencephalographic activation associated with cerebral cortical arousal, this occurs while the animals appear to be awake with full postural control, suggesting that noncholinergic pathways to the cerebral cortex are also involved in such behavioral manifestations. The roles of acetylcholine in cognitive processes such as attention, learning, memory, responses to environmental changes, and motor activity still remain to be defined.