Introduction

Fast synaptic signaling in the vertebrate retina encodes presynaptic voltages as time-varying modulations in extracellular neurotransmitter concentrations and decodes neurotransmitter signals with postsynaptic transmembrane ionotropic or heptahelical receptor arrays. Additional heptahelical receptor pathways conditionally modulate synaptic signaling, often on lower temporal and spatial frequency scales. The six major retinal neurotransmitters—glutamate, gamma-aminobutyric acid (GABA), glycine, acetylcholine, dopamine, and serotonin—are synthesized in group-transfer reactions associated with distinctive cells. The sources and targets of less circumscribed, small-molecule modulators are mentioned only briefly, and peptides are treated elsewhere. Vertical channels deploy fast, high-gain glutamatergic synapses between photoreceptors (PRs) and their bipolar cell (BC) and horizontal cell (HC) targets, and between bipolar cells and their amacrine cell (AC) and ganglion cell (GC) targets. Lateral channels composed of horizontal cells and amacrine cells primarily use fast, low-gain, sign-inverting GABAergic and glycinergic synapses, with specialized circuits employing high-gain, sign-conserving cholinergic signaling. Four modes of fast synaptic transfer dominate neuronal receptive field circuits: high-gain, sign-conserving (⇒) or sign-inverting (i⇒) transfers, and low-gain, sign-conserving (→) or sign-inverting (i→) transfers. The following sections summarize metabolic networks, transporters, receptors, and circuitries for each neurotransmitter species, concluding with a summary of future directions. Figures 20.1 to 20.6 encapsulate metabolic network diagrams, Enzyme Commission (EC) and Transport Commission (TC) codes, localizations of macromolecules associated with a neurotransmitter phenotype, and other data. References are restricted to reviews or recent exemplars of concepts from which original literature may be traced.

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