In response to light, what happens to cGMP levels in photoreceptor cells?

In response to light, what happens to cGMP levels in photoreceptor cells?

• A. They increase
• B. They remain unchanged
• C. They decrease
• D. They are converted to cAMP

Answer: (C)

Light triggers phototransduction by lowering the level of cGMP in photoreceptor cells. When photons are absorbed, rhodopsin activates a G protein that, in turn, stimulates phosphodiesterase to hydrolyze cGMP to GMP. With less cGMP available, the cGMP-gated ion channels close, reducing Na+ and Ca2+ influx. This causes the cell to hyperpolarize and decreases glutamate release onto downstream neurons. In darkness, cGMP is high and these channels stay open, keeping the cell depolarized. So the correct outcome is a decrease in cGMP levels in response to light, rather than an increase, no change, or conversion to cAMP.

Light triggers phototransduction by lowering the level of cGMP in photoreceptor cells. When photons are absorbed, rhodopsin activates a G protein that, in turn, stimulates phosphodiesterase to hydrolyze cGMP to GMP. With less cGMP available, the cGMP-gated ion channels close, reducing Na+ and Ca2+ influx. This causes the cell to hyperpolarize and decreases glutamate release onto downstream neurons. In darkness, cGMP is high and these channels stay open, keeping the cell depolarized. So the correct outcome is a decrease in cGMP levels in response to light, rather than an increase, no change, or conversion to cAMP.