Visual cascade

The G-proteins are heterotrimers made of three families of subunits, a, P, and y, which can interact specifically with discrete regions on G-protein-coupled receptors. This includes most receptors for neurotransmitters and polypeptide hormones (see Neuroregulators). G-protein-coupled receptors also embrace the odorant receptor family and the rhodopsin-linked visual cascade. 

It is instructive to note that the biochemistry of the reactions that initiate the visual cascade and the glycogenolytic cascade is similar. The cyclic AMP-dependent protein kinase complex comprises the regulatory and catalytic components (R and C) for which the regulatory signal is the concentration of cyclic AMP. This binds to the regulatory component of the kinase (the R subunit) which then dissociates from the R-C complex. The C is now catalyti-cally active and catalyses the initial reaction in a cascade sequence which leads to activation of the target protein (phosphorylase). 

For termination of the visual cascade, Rh is phosphorylated by the rhodopsin kinase which allows subsequent binding of arrestin molecules to P-Rh inhibiting further R -transducin interaction. CNG channels-closing leads to a decrease of the free Ca " " concentration and an activation of Ca " -dependent proteins in the outer segment. In mm these proteins activate guanylate cyclases (GC) and lead to a delayed restoration of the cGMP concentration (Nakatani et al. 2002). 

Light-induced isomerization of 11-cts-retinal plays a central role in the conversion of visual signals, i.e., light, into nerve impulses known as the visual cascade (see below) (23). 

The effect of receptor stimulation is thus to catalyze a reaction cycle. This leads to considerable amplification of the initial signal. For example, in the process of visual excitation, the photoisomerization of one rhodopsin molecule leads to the activation of approximately 500 to 1000 transdudn (Gt) molecules, each of which in turn catalyzes the hydrolysis of many hundreds of cyclic guanosine monophosphate (cGMP) molecules by phosphodiesterase. Amplification in the adenylate cyclase cascade is less but still substantial each ligand-bound P-adrenoceptor activates approximately 10 to 20 Gs molecules, each of which in turn catalyzes the production of hundreds of cyclic adenosine monophosphate (cAMP) molecules by adenylate cyclase. 

Rdzanowska, M and Rozanowski, B, 2008. Visual transduction and age-related changes in lipofuscin. In Tombran-Tink, J and Barnstable, CJ (Eds.), Ophthalmology Research The Visual Transduction Cascade. The Humana Press Inc., Totowa, NJ, pp. 405 146. 

Figure 29-5. The visual cycle of vitamin A is central to vision. In the retina, light stimulates the conversion of 1 l-c/.s-retinal, part of rhodopsin (Rho), to all-iraws-retinal and activates rhodopsin (Rho ).This initiates the first step of the signal transduction cascade that results in the transmission of the visual signal to the brain.The visual cycle involves biochemical and metabolic events in both the photoreceptors (rods) and the retinal pigment epithelium.

In both types of photoreceptor cells, the outer segment contains all components of the visual transduction cascade which is one of the best studied examples of... 

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