Problems and ambiguities in the identification of autonomic neurotransmitters

Although the traditional classification of autonomic neurons as either cholinergic or adrenergic is still subscribed to in most textbooks of physiology, it is apparent that many of the postganglionic neurons do not fall readily into either of these categories. Upwards of a dozen other transmitter candidates have been proposed, but in no instance has the identity of a non-cholinergic, non-adrenergic autonomic transmitter been established. In this paper some of the problems and ambiguities relating to satisfaction of the classical criteria for neurotransmitter identification are briefly surveyed, with special reference to the neuropeptides.