Phototransduction in rod photoreceptors involves a G-protein cascade. In the dark, cGMP-gated channels maintain a 'dark current.' When light is absorbed by rhodopsin, the sequence of events that closes these channels is:

• A Rhodopsin* → Gq → phospholipase C → IP3 → Ca2+ release → CNG channel closure
• B Rhodopsin* → transducin (Golf/Gt) → PDE6 activation → cGMP hydrolysis → CNG channel closure → hyperpolarization ✓
• C Rhodopsin* → Gs → adenylyl cyclase → cAMP → PKA phosphorylation of CNG channels causing closure
• D Rhodopsin* → Gi → adenylyl cyclase inhibition → reduced cAMP → K+ channel opening → hyperpolarization

Explanation

Light activates rhodopsin (R*), which activates the rod-specific G-protein transducin (Gt, comprising Gt-alpha-GTP). Active Gt-alpha activates phosphodiesterase 6 (PDE6), which rapidly hydrolyzes cGMP to 5'-GMP. In darkness, high cGMP keeps CNG (cyclic nucleotide-gated) cation channels open, allowing an inward dark current (Na+ and Ca2+). When cGMP falls, CNG channels close, reducing Ca2+ influx, hyperpolarizing the rod cell, and reducing glutamate release onto bipolar cells — the electrical signal of visual transduction. Recovery is mediated by guanylyl cyclase activation (via GCAP proteins sensing reduced Ca2+), which resynthesizes cGMP.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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