This paper was presented at a colloquium entitled ‘ ‘ Vision : From Photon to Perception , ’ ’ organized by

Recent studies have elucidated how the absorption of a photon in a rod or cone cell leads to the generation of the amplified neural signal that is transmitted to higher-order visual neurons. Photoexcited visual pigment activates the GTP-binding protein transducin, which in turn stimulates cGMP phosphodiesterase. This enzyme hydrolyzes cGMP, allowing cGMP-gated cationic channels in the surface membrane to close, hyperpolarize the cell, and modulate transmitter release at the synaptic terminal. The kinetics of reactions in the cGMP cascade limit the temporal resolution of the visual system as a whole, while statistical f luctuations in the reactions limit the reliability of detection of dim light. Much interest now focuses on the processes that terminate the light response and dynamically regulate amplification in the cascade, causing the single photon response to be reproducible and allowing the cell to adapt in background light. A lightinduced fall in the internal free Ca21 concentration coordinates negative feedback control of amplification. The fall in Ca21 stimulates resynthesis of cGMP, antagonizes rhodopsin’s catalytic activity, and increases the affinity of the lightregulated cationic channel for cGMP. We are using physiological methods to study the molecular mechanisms that terminate the f lash response and mediate adaptation. One approach is to observe transduction in truncated, dialyzed photoreceptor cells whose internal Ca21 and nucleotide concentrations are under experimental control and to which exogenous proteins can be added. Another approach is to observe transduction in transgenic mouse rods in which specific proteins within the cascade are altered or deleted. Vision begins with the conversion of light from the outside world into electrical signals which can be processed within the retina and sent to the brain. When the conversion fails, as it does in hereditary retinal degenerations, a willing brain is left unable to see. The workings of the first step fix the absolute sensitivity, spectral sensitivity, and temporal resolution of the visual system as a whole. Our understanding of the molecular basis of visual transduction has deepened rapidly in recent years as physiology, biochemistry, and molecular biology have been brought to bear. Insights gained from the study of transduction in photoreceptor cells have helped to elucidate signaling in a wide variety of other cell types that use Gprotein-coupled receptors and cyclic nucleotide cascades. My aim in this article is to review some of the accomplishments and gaps in our understanding of visual signal generation. Electrical and Chemical Signaling in Rods and Cones Rods and cones have the structure diagramed in Fig. 1 A. The outer segment, containing the visual pigment, is connected to the inner segment, which bears a synaptic terminal contacting bipolar and horizontal cells. Light absorbed in the pigment acts to close cationic channels in the outer segment, causing the surface membrane of the entire cell to hyperpolarize. The hyperpolarization relays visual information to the synaptic terminal, where it slows ongoing transmitter release. The cationic channels in the outer segment are controlled by the diffusible cytoplasmic ligand cGMP, which binds to channels in darkness to hold them open. Light closes channels by lowering the cytoplasmic concentration of cGMP. The steps that link light absorption to channel closure in a rod are illustrated schematically in Fig. 1B. When rhodopsin (R) absorbs a photon its 11-cis-retinal chromophore rapidly isomerizes, causing the cytoplasmic surface of the protein to become catalytically active. In this state, rhodopsin activates the GTP-binding protein transducin (T). Within a fraction of a second a single active R causes hundreds of transducins to exchange bound GDP for GTP, forming active TGTP complexes. A greatly amplified signal now passes to a third protein, cGMP PDE, which is activated by TGTP. Activated PDE hydrolyzes cGMP to 59-GMP, which cannot open the channel. With cGMP removed, channels close, interrupting a steady inward current of Na1 and Ca21, thus hyperpolarizing the cell. These activation steps in transduction are now well established (see ref. 1 for review) and their behavior has been described quantitatively (2). The events that terminate the response to light are not so well understood. Catalytically active rhodopsin is thought to be shut off by phosphorylation followed by binding of the soluble protein arrestin. The time course of shutoff in vivo as well as the relative importance of phosphorylation and arrestin binding in reducing rhodopsin’s catalytic activity are not yet known however. Active transducin is thought to be shut off by hydrolysis of the GTP bound to it. Although this process proceeds slowly in the test tube, heat measurements on outer segment preparations indicate that it occurs on the subsecond time scale expected from the time course of the electrical response to light (3). In the intact outer segment the GTPase activity of transducin may be accelerated by a specific protein. A candidate for the accelerator is the g subunit of PDE (4), which may act in concert with another membrane-bound protein (5). PDE shuts off when its inhibitory subunit, freed by deactivation of TGTP, recombines with the catalytic subunits. Finally, the cGMP concentration is restored to the dark level by cGMP synthesis, which is mediated by guanylate cyclase. Single Photon Effect Pioneering psychophysical experiments half a century ago indicated that a retinal rod registers the absorption of a single photon, the smallest unit of light energy (6). Electrical recordings confirm this behavior and reveal that the elementary response is highly amplified. In a mammalian rod, for example, the quantal response has a peak amplitude of about 1 pA and over the entire response the entry of about one million The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked ‘‘advertisement’’ in accordance with 18 U.S.C. §1734 solely to indicate this fact. Abbreviation: PDE, phosphodiesterase.