Which molecule decreases in concentration during phototransduction, facilitating the response of photoreceptors to light?

During phototransduction, the primary process by which photoreceptors (rods and cones) convert light into electrical signals, the concentration of cyclic guanosine monophosphate (cGMP) decreases. In the dark, cGMP levels are maintained high, allowing for the opening of cyclic nucleotide-gated ion channels on the photoreceptor cell membranes. This results in an influx of sodium and calcium ions, leading to the depolarization of the photoreceptors and the release of neurotransmitters that signal to bipolar cells.

However, when light photons strike the photopigments in the photoreceptors, a cascade of events is triggered that activates the enzyme phosphodiesterase. This enzyme hydrolyzes cGMP into GMP, thereby reducing the concentration of cGMP in the cell. As the levels of cGMP drop, the cyclic nucleotide-gated ion channels close, preventing sodium and calcium from entering the cells. This leads to hyperpolarization of the photoreceptor and reduced neurotransmitter release, effectively signaling to the bipolar cells that light has been detected.

Thus, cGMP is an essential molecule in the phototransduction pathway, and its decrease during illumination is crucial for the proper response of photoreceptors