Phototransduction causes a decrease in the concentration of which second messenger?

Phototransduction causes a decrease in the concentration of which second messenger?

• A. cAMP
• B. IP3
• C. DAG
• D. cGMP

Answer: (D)

In phototransduction, the key second messenger is cGMP. In darkness, high cGMP keeps cyclic nucleotide–gated channels open, allowing a steady inward current that keeps the cell depolarized. When light activates the visual pigment, the cascade activates phosphodiesterase, which rapidly hydrolyzes cGMP to GMP. The drop in cGMP causes those channels to close, the cell hyperpolarizes, and transmitter release decreases. The other messengers listed—cAMP, IP3, and DAG—aren’t involved in this rod pathway, so their levels aren’t the ones that decrease in response to light.

In phototransduction, the key second messenger is cGMP. In darkness, high cGMP keeps cyclic nucleotide–gated channels open, allowing a steady inward current that keeps the cell depolarized. When light activates the visual pigment, the cascade activates phosphodiesterase, which rapidly hydrolyzes cGMP to GMP. The drop in cGMP causes those channels to close, the cell hyperpolarizes, and transmitter release decreases. The other messengers listed—cAMP, IP3, and DAG—aren’t involved in this rod pathway, so their levels aren’t the ones that decrease in response to light.