Chemical Coding of Neurons and Plurichemical Transmission

Abstract

Two unexpected facts about neurons have been established in recent years: neurons contain and can release a large variety of substances that are capable of influencing target cells; and it is common for individual neurons to contain several such substances. Both of these findings have opened the way to the solution of old problems and have posed problems of their own. The discov­ ery of a cornucopia of substances contained in neurons has led to a revitaliza­ tion of microscopic neuroanatomy, spawning a new field termed neuroanatomy (1). This burgeoning of knowledge of immunohistochemi­ cally identifiable neuronal markers occurred in the last decade and now encompasses thousands of publications, numerous reviews, several books, and even a Handbook series and a journal (1-8). On the other hand, it has been a considerably more difficult task for physiologists and pharmacologists to evaluate the roles of these substances and, in particular, to determine in which ways more than one substance can participate in the transmission process. In the present review we look at two aspects of recent work in this field: the ways in which combinations of neuronal markers identify individual groups of neurons. a phenomenon referred to here as chemical coding; and the manner in which neurotransmission can involve the combined release and action of more than one substance. referred to as plurichemical transmission.