What is the first step in the phototransduction mechanism?

What is the first step in the phototransduction mechanism?

• A. Light activation causes photopigment to change conformation.
• B. Sodium channels begin to open.
• C. cGMP is hydrolyzed.
• D. Glutamate release increases.

Answer: (A)

The initial event in phototransduction is photon absorption by the photopigment, which causes the chromophore retinal to isomerize and the photopigment to change its conformation to the activated form. That conformational change is what starts the whole signaling cascade: it activates the G-protein transducin, which then stimulates phosphodiesterase to hydrolyze cGMP, leading to closure of cGMP-gated sodium channels and hyperpolarization of the photoreceptor. The other steps come later or under different conditions—sodium channels open in the dark when cGMP is high, cGMP hydrolysis happens downstream of the photopigment activation, and glutamate release increases in darkness and falls with light.

The initial event in phototransduction is photon absorption by the photopigment, which causes the chromophore retinal to isomerize and the photopigment to change its conformation to the activated form. That conformational change is what starts the whole signaling cascade: it activates the G-protein transducin, which then stimulates phosphodiesterase to hydrolyze cGMP, leading to closure of cGMP-gated sodium channels and hyperpolarization of the photoreceptor. The other steps come later or under different conditions—sodium channels open in the dark when cGMP is high, cGMP hydrolysis happens downstream of the photopigment activation, and glutamate release increases in darkness and falls with light.