Phototransduction Cascade

Rhodopsin consists of a chromophore, 11-cis retinal (the aldehyde form of vitamin A), and a protein, opsin (Fig. 4). Absorption of a photon of light changes the chromophore to the all-trans form of retinal. This con-formational change produces a short-lived intermediate, meta-rhodopsin II, which activates a G-protein called transducin. Transducin activates a phosphodiesterase enzyme, leading to a reduction in the intracellular levels of cyclic guanosine monophosphate (cGMP). As cGMP

levels are lowered, the conductance of sodium channels in the rod plasma membranes is decreased, and the membrane becomes hyperpolarized in proportion to the amount of light initially absorbed.

It may initially seem counterintuitive for rods to become hyperpolarized in response to increased light exposure; however, it seems less so in light of the fact that rods do not have action potentials. Sodium channels of photoreceptors are not voltage dependent, so their conductances are regulated almost entirely through the cGMP regulatory site. Rods are capable of responding to both increases and decreases in light by producing graded potentials that reflect an increase or decrease in the levels of cGMP. The final output of the rod is in the form of an increase or decrease in the amount of transmitter (glutamate) released at the photoreceptor synapse.

The sodium channels of rods also gate calcium ions that provide a feedback loop to regulate cGMP synthesis. In low levels of light, cGMP levels are high, channels are open, and sodium and calcium can enter the cell. The inward sodium current (called the dark current) keeps the cell relatively depolarized; in addition, the influx of calcium inhibits guanylate cyclase activity. Because both synthetic and degradative enzymes for cGMP are relatively inactive under these conditions, cGMP levels remain fairly constant. An increase in light decreases cGMP levels and causes a rapid decrease in the conductance of both sodium and calcium.

51. The Visual System rod photoreceptor

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