Describe the phototransduction cascade activated by light in photoreceptors leading to hyperpolarization.

• A. Light activates rhodopsin → transducin activates PDE → cGMP decreases → cGMP-gated Na+ channels close → hyperpolarization
• B. Light activates rhodopsin → transducin activates PDE → cGMP increases → channels open
• C. Light activates rhodopsin → transducin inhibits PDE → cGMP increases → channels open
• D. Light activates rhodopsin → transducin activates adenylate cyclase → cAMP → channels open

Answer: (A)

Light hitting photoreceptors triggers a G-protein–coupled cascade that leads to hyperpolarization. When light activates the photopigment rhodopsin, it activates the G protein transducin. Transducin then activates phosphodiesterase, which hydrolyzes cGMP, lowering its intracellular level. The cGMP-gated Na+ channels depend on cGMP to remain open; with less cGMP, these channels close, reducing Na+ (and Ca2+) influx and causing the cell membrane to hyperpolarize. This decreased inward current also reduces glutamate release at the synapse. The other options describe either an increase in cGMP and channel opening, inhibition of PDE, or involvement of adenylate cyclase and cAMP, none of which match the light-activated rod phototransduction pathway.