8 - Transmitters and Receptors

Abstract

Electrical synapses are gap junctions designed to ensure synchronous activity of groups of neurons. The gaps are bridged by tightly packed ion channels. Protein subunits forming the individual ion channels are closed when the neurons are silent. The channels open in response to specific stimuli (such as action potentials), allowing instant diffusion of ions directly from one cytosol to another. At chemical synapses, transmitter molecules are expelled into the synaptic cleft and bind to their specific target receptors. Ionotropic receptors are ligand gated. Each is either excitatory (allowing entry of Na+ ions) or inhibitory (allowing passage of Cl− or K+ ions). Metabotropic receptors are transmembrane proteins without an ion pore. Their receptor macromolecule responds to transmitter activation by detaching a G protein subunit that usually binds to guanine triphosphate or guanine diphosphate, which in turn activates the cyclic AMP (cAMP), phosphoinositol, or arachidonic acid system. These second messengers interact with intracellular kinases and proteins to alter the membrane potential of the target neuron. Amino acid transmitters include glutamate, GABA (γ-aminobutyric acid), and glycine. Biogenic amine transmitters and modulators include acetylcholine (ACh) and the monoamines (i.e. the catecholamines (dopamine, norepinephrine, and epinephrine), serotonin, and histamine). Neuropeptides include vasoactive intestinal polypeptide (VIP), substance P, enkephalin, and endorphins. Also prevalent are adenosine and nitric oxide.