During the dark state of phototransduction, which description applies to photoreceptors?

During the dark state of phototransduction, which description applies to photoreceptors?

• A. cGMP is synthesized continuously, Na+ channels are open, and glutamate is released.
• B. cGMP is hydrolyzed, Na+ channels close, and glutamate release increases.
• C. cGMP is hydrolyzed, channels open, causing depolarization.
• D. There is no change in photoreceptor activity.

Answer: (A)

In the dark, photoreceptors stay depolarized because cGMP levels remain high. Guanylate cyclase keeps producing cGMP, and this cGMP keeps the cyclic nucleotide-gated channels open, allowing Na+ (and Ca2+) to flow into the cell. That steady inward current keeps the membrane at a relatively depolarized level and drives continuous glutamate release at the synapse onto downstream retinal neurons. In bright light, a cascade activates phosphodiesterase to hydrolyze cGMP, closing those channels, causing hyperpolarization and a drop in glutamate release. So the description that cGMP is synthesized continuously, Na+ channels are open, and glutamate is released accurately reflects the dark state.

In the dark, photoreceptors stay depolarized because cGMP levels remain high. Guanylate cyclase keeps producing cGMP, and this cGMP keeps the cyclic nucleotide-gated channels open, allowing Na+ (and Ca2+) to flow into the cell. That steady inward current keeps the membrane at a relatively depolarized level and drives continuous glutamate release at the synapse onto downstream retinal neurons. In bright light, a cascade activates phosphodiesterase to hydrolyze cGMP, closing those channels, causing hyperpolarization and a drop in glutamate release. So the description that cGMP is synthesized continuously, Na+ channels are open, and glutamate is released accurately reflects the dark state.