The concept of chemical transmission in the central nervous system has taken some time to be generally accepted, but an increasing number of compounds are now being recognized as hav- ing a transmitter role in the brain. The acetylcholine system was the first to be discovered in the periphery and its charac- teristic features of storage of transmitter in vesicles in the nerve terminal, its electrically-evoked release and rapid extra- neuronal breakdown were considered to be necessary criteria for any neurotransmitter candidate. The subsequent elucidation of the noradrenergic system made it...