In darkness D, left), cGMP is bound to cation channels (Na+, Ca2+) in the outer segment of the photosensor, thereby keeping them open. Na+ and Ca2+ can therefore enter the cell and depolarize it to about -40mV (^ D3, D4). This darkness-induced influx into the outer segment is associated with the efflux of K+ from the inner segment of the sensor. The Ca2+ entering the outer segment is immediately transported out of the cell by a 3 Na+/Ca2+ exchanger (^ p. 36), so the cytosolic Ca2+ concentration [Ca 2+]i remains constant at ca. 350500 nmol/L in darkness (^ D6). If the cytosolic cGMP concentration decreases in response to a light stimulus (^ D2), cGMP dissociates from the cation channels, allowing them to close. The photosensor then hyperpolarizes to ca. - 70 mV (sensor potential: ^ D, right). This inhibits the release of glutamate (transmitter) at the sensor pedicle (^ D5), which subsequently causes changes in the membrane potential in downstream retinal neurons (^ p. 354).

Deactivation of Photic Reactions and Regeneration Cycles

♦ Rhodopsin (^ E2). Rhodopsin kinase (RK) competes with transducin for bindings sites on metarhodopsin II (MR-II); the concentration of transducin is 100 times higher (^ E2, right). Binding of RK to MR-II leads to phosphorylation of MR-II. As a result, its affinity to trans-ducin decreases while its affinity to another protein, arrestin, rises. Arrestin blocks the binding of further transducin molecules to MR-II. All-trans-retinal detaches from opsin, which is subsequently dephosphorylated and re-loaded with 11-cis-retinal.

♦ All-trans-retinal (^ E1) is transported out of the photosensor and into the pigmented epithelium, where it is reduced to all-trans-retinol, esterified and ultimately restored to 11-cis-retinal. After returning into the photosensor, it binds to opsin again, thereby completing the cycle (^ E2).

♦ Note: Retinol is vitamin A1. A chronic deficiency of vitamin A1 or its precursors (carotinoids) leads to impaired rhodopsin production and, ultimately, to night blindness (^ p. 352).

♦ Transducin (^ E3). Since the GTPase activity of as-GTP breaks down GTP into GDP + Pi, the molecule deactivates itself. The as-GTP molecule and subunit then reunite to trans-

ducin. GAP (GTPase-activating protein) accelerates the regeneration of transducin. Phos-ducin, another protein, is phosphorylated in the dark(^ D6) and dephosphorylated in light (^ D7). The latter form binds to the subunit (^ D7, E3), thereby blocking the regeneration of transducin. This plays a role in light adaptation (see below).

♦ Phosphodiesterase (PDE). In the course of transducin regeneration, the inhibitory subunit of cGMP phosphodiesterase (Ipde) is released again and PDE is thus inactivated.

♦ cGMP. Since the 3Na+/Ca2+ exchanger still functions even after photostimulation-in-duced closure of Ca2+ channels, the [Ca2+]i starts to decrease. When a threshold of ca. 100 nmol/L is reached, the Ca2+-binding protein GCAP (guanylyl cyclase-activating protein) loses its 4 Ca2+ ions and stimulates guany-lyl cyclase, thereby accelerating cGMP synthesis. Thus, the cGMP concentration rises, the cation channels re-open, and the sensor is ready to receive a new light stimulus. This Ca2+ cycle therefore mediates a negative feedback loop for cGMP production.

Ca2+ Ions and Adaptation (see also p. 352) In the dark, the [Ca2+]i is high, and calmodulin-bound Ca2+ (^ p. 36) stimulates the phospho-rylation ofphosducin with the aid ofcAMP and phosphokinase A (^ D6). In light, the [Ca2+]i is low; phosducin is dephosphorylated and rapid regeneration of transducin is not possible (^ D7, E3). Moreover, Ca2+ accelerates the phosphorylation of MR-II in light with the aid of another Ca2+ binding protein, recoverin (^ E2). Ca2+ is therefore essential for the adaptation of photosensors (^ p.352).

Although they contain similar enzymes and transmittes, the photosensitivity of the cones is about 100 times less than that of the rods. Thus, the cones are unable to detect a single quantum of light, presumably because photic reactions in the cones are deactivated too quickly. Compared to rhodopsin in the retinal rods, the three visual pigments in the three types of cones (11-cis-retinal with different opsin fractions) only absorb light in a narrow wavelength range (^ p. 357 E), which is a prerequisite for color vision (^ p. 356).

|— D. Activation of photosensors and adaptation

Rhodopsin^ 1

Disk membrane

Transducin ß y

Disk membrane

ICF of rod

MR-II (metarhodopsin II) ß


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