Changing concepts about neuroregulation: neurotransmitters and neuromodulators

Abstract

Recently, discoveries of an astounding number of endogenous substances which affect interneuronal communication, and advances in our understanding of mechanisms by which it is controlled, have created a growing need for a better classification of compounds which are involved in neuronal transmission. In this paper we describe a nomenclature based on the common ability of such neuroactive substances to regulate neuronal communication. We suggest that 'neuroregulator' is a good generic term, since it focuses on the relevant activity. This can be further subdivided according to function. Thus, a 'neurotransmitter' would correspond to the classical concept of a substance which conveys a transient and unilateral signal across a specialized synapse. In contrast, a 'neuro-modulator' would alter neuronal activity by mechanisms which might or might not involve a synapse. Two types of neuro-modu-lators might be: hormonal neuromodulators, providing direct short-or long-lasting modulation of neurons far from the release site, and synaptic neuromodulators, acting indirectly by modulating neuro-transmitter function. Preliminary criteria for classifying neuroregu-lators are described. As discussed in the paper, introduction of a nomenclature for neuroregulators could have several salutory effects. For example, it 43