Q.91.Visual signal transduction cascade is activated by rhodopsin and involves degradation rather than
synthesis of which ONE of the following second messenger molecules?
(A) cAMP
(B) IP3
(C) cGMP
(D) DAG
Visual signal transduction in rod cells begins when light activates rhodopsin, triggering a cascade that degrades cGMP as the key second messenger. The correct answer to the query is (C) cGMP, as its hydrolysis closes ion channels and hyperpolarizes the cell. This process contrasts with synthesis-dominated pathways in other systems.
Cascade Overview
Light isomerizes retinal in rhodopsin, forming metarhodopsin II that activates transducin. GTP-bound transducin stimulates phosphodiesterase (PDE), which rapidly hydrolyzes cGMP to GMP. Reduced cGMP closes cyclic nucleotide-gated channels, decreasing Na⁺ influx and hyperpolarizing the photoreceptor.
Option Explanations
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(A) cAMP: Functions in many GPCR pathways via adenylyl cyclase synthesis, but plays no direct role in vertebrate phototransduction; early studies dismissed it.
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(B) IP3: Generated with DAG from PIP2 by PLC in Gq pathways, involved in Ca²⁺ release, but absent in rhodopsin-transducin cascade.
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(C) cGMP: Correct; light increases PDE activity, degrading cGMP levels from 5-10 μM to sub-micromolar in milliseconds.
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(D) DAG: Produced alongside IP3, activates PKC in some systems, but not part of visual transduction.
Exam Relevance
This mechanism appears in exams like GATE Life Sciences, emphasizing cGMP’s unique degradation in vision versus synthesis elsewhere. Guanylate cyclase restores cGMP in darkness for recovery.
