Neurotransmitters as the Chemical Messengers of Certain Circuits and Pathways

Abstract

Communication between neurons occurs primarily through the release of neuroactive chemical messengers called neurotransmitters or neuromodulators (Chap. 3). For a chemical agent to be called a neurotransmitter, it must be synthesized in the presynaptic neuron (acetylcholine and the monoamines are produced in the axon terminals), be stored in presynaptic vesicles and released into a synaptic cleft, bind to a receptor binding site on the postsynaptic membrane of another neuron or effector (muscle fiber or gland cell) where it regulates ion channels, and alter the membrane potential. Finally, it must be removed from the synaptic cleft by reuptake into the presynaptic neuron or by biochemical degradation. Those neuroactive substances that have not been demonstrated to fulfill all of these requirements are often called putative neurotransmitters. Some investigators define transmitters as chemical messengers that interact with receptors directly linked to channel proteins, and neuromodulators as chemical messengers that interact with a receptor linked to a G-protein and a second-messenger system (Chap. 3).