Monoamines and the Orchestration of Behavior

Abstract

A ll animal behavior is the outcome of the orches- trated activity of large assemblies of neurons in the central nervous system. Within such an assembly, each neuron can be recruited by chemical compounds known as neurotransmitters, neuromodulators, and neurohormones. Neurotransmitter molecules are released into the synapse, where they act directly on receptor channels in the cell membrane and produce rapid opening of these channels for brief periods, at a time scale of milliseconds. This induces a rapid change in the electrical activity of neurons. Neuromodulators are released into a broader area than are neurotransmitters, but they are nevertheless targeted. Like neurotransmitters, neuromodulators bind to receptors in the cell membrane, but these receptors are generally linked to G proteins that activate intracellular signaling cascades, with effects on the electrical activity of neurons lasting seconds, minutes, hours, days, or even weeks. Neuromodulators have a wide variety of effects at the molecular level, including (a) modulation of ion channels and receptors and (b) changes in protein synthesis, enzyme activity, and gene transcription. Unlike the other two categories of neuroactive substances, neurohormones are circulated in the blood or hemolymph and thus act globally within the organism.