Visual signal transduction

One of the few examples for which the amplification factor at the level of the activated receptor could be determined is for the visual signal transduction pathway (review Lamb, 1996). 

P D E6 is by far the dominant P D E occurring in rod and cone cells of the vertebrate retina. It plays a key role in visual signal transduction, which is unique among the... 

Fig. 2. The role of rhodopsin in visual signal transduction (see text). (A) Activation. (B) Inactivation.

Conceptual Insights, Signaling Pathways Response and Recovery presents an animated version of Figure 32.5 and a comparison to visual signal transduction (Figure 32.5). 

The retinal pool of 11 -cis-retinal is clearly critical for photon detection and visual signal transduction. There are two key aspects regarding this that are important to consider the precursor(s) from which 11 -cis-retinal derived, and, bioconversion of isomerized trans-retinal back to 11-cis-retinal that can recombine with opsin to re-form rhodopsin. The latter aspect is obligatory for restoration of the dark state, regenerating a photosensitive receptor capable of undergoing another cycle of photon detection and signal transduction. Considered together, these events represent the retinoid cycle in the visual process (22). The entire process of re-isomerization and formation of a new rhodopsin molecule occurs in two different retinal tissues and involves several enzymatic steps that are described below (see Fig. 7.14). 

Figure 32.25 Visual signal transduction. The light-induced activation of rhodopsin leads to the hydrolysis of cGMP, which in turn leads to ion-channel closing and the initiation of an action potential.

In visual signal transduction, activation proceeds from the receptor, rhodopsin, to the effector, phosphodiesterase (PDE), via the visual G protein, Gp Each Mil sequentially binds and activates up to 100 Gj, thus MII-Gj binding initiates the first stage of signal amplification in the visual pathway. Litman et al. (2001) have studied the phospholipid acyl chain dependence of the kinetics of formation of both the Mil conformation and the... 

Fig. 4. Examples of the effects of acyl chain composition (white bars) and cholesterol (gray bars) on A eq for the MI-MII equilibrium of photolyzed rhodopsin at 37°C. Higher values of correspond to higher equilibrium concentrations of Mil, the state of photolyzed rhodopsin that participates in visual signal transduction by binding the visual G protein.

The role of retinal (18, vitamin A aldehyde) in the visual process, involving cis/trans isomerisation of the sterically hindered C-11,12 double bond, is well established [28,29]. Besides the important function of retinal in visual signal transduction in animals is the function of energy production in halophilic bacteria, where the retinal-based bacterio-rhodopsin takes part in a light driven proton pump [30]. 

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