Describe the rod phototransduction cascade moving from dark to light.

• A. In darkness, rhodopsin is inactive; high cGMP; cGMP-gated channels open; rod depolarizes; glutamate release.
• B. In darkness, rhodopsin is active; low cGMP; channels close; rod hyperpolarizes.
• C. In light, rhodopsin activates transducin; cGMP increases; channels open; depolarization.
• D. In light, photoreceptors release more glutamate.

Answer: (A)

The key idea is how rod cells behave in the dark as the starting point for the light response. In darkness, rhodopsin is not active, so the G-protein cascade that would normally quench the current isn’t engaged. This leaves a high level of cGMP inside the outer segment. The cGMP-gated cation channels stay open, allowing Na+ (and some Ca2+) to enter and produce a steady depolarizing current—the dark current. Because the cell is depolarized, it continuously releases glutamate onto downstream neurons.

When light arrives, rhodopsin is activated and turns on the transducin pathway. Activated transducin stimulates phosphodiesterase, which lowers cGMP levels. With less cGMP, the channels close, the inward current drops, and the rod hyperpolarizes. This hyperpolarization reduces glutamate release, signaling the presence of light to the retina.

So describing the dark state as high cGMP with open channels and depolarization that drives glutamate release is the correct baseline for understanding what changes as you move toward the light response.