What is the role of phosphodiesterase in phototransduction?

What is the role of phosphodiesterase in phototransduction?

• A. It synthesizes cGMP.
• B. It activates Na+ channels.
• C. It hydrolyzes cGMP, leading to a decrease in cGMP levels.
• D. It phosphorylates rhodopsin.

Answer: (C)

In phototransduction, the level of cGMP inside the photoreceptor determines whether the cGMP-gated channels stay open. Light-activated rhodopsin triggers a G protein that activates phosphodiesterase, which hydrolyzes cGMP to GMP, lowering cGMP levels. With less cGMP, the channels close, reducing the inward Na+ and Ca2+ current and causing the cell to hyperpolarize. This hyperpolarization decreases neurotransmitter release, signaling light detection downstream. This action is distinct from synthesizing cGMP (that’s done by guanylate cyclase) or directly opening Na+ channels, and it doesn’t involve rhodopsin phosphorylation (that’s done by rhodopsin kinase/arrestin).

In phototransduction, the level of cGMP inside the photoreceptor determines whether the cGMP-gated channels stay open. Light-activated rhodopsin triggers a G protein that activates phosphodiesterase, which hydrolyzes cGMP to GMP, lowering cGMP levels. With less cGMP, the channels close, reducing the inward Na+ and Ca2+ current and causing the cell to hyperpolarize. This hyperpolarization decreases neurotransmitter release, signaling light detection downstream. This action is distinct from synthesizing cGMP (that’s done by guanylate cyclase) or directly opening Na+ channels, and it doesn’t involve rhodopsin phosphorylation (that’s done by rhodopsin kinase/arrestin).