Does light absorption increase or decrease the concentration of cGMP in photoreceptors?

In photoreceptors, specifically in rod and cone cells, the absorption of light is crucial for the phototransduction cascade, which converts light signals into electrical signals. When photons are absorbed by the photopigment (rhodopsin in rods and photopsins in cones), it triggers a biochemical cascade that leads to the closure of ion channels in the photoreceptor membrane.

Under dark conditions, cGMP (cyclic guanosine monophosphate) is kept at relatively high levels, which keeps certain cation channels open, allowing the influx of sodium ions and thus maintaining a depolarized state of the cell. However, when light is absorbed, the phototransduction cascade activates phosphodiesterase (PDE), an enzyme that breaks down cGMP into GMP. As a result, the concentration of cGMP decreases.

This decrease in cGMP leads to the closure of the cation channels, which in turn leads to hyperpolarization of the photoreceptor cell. This hyperpolarization causes a decrease in the release of neurotransmitters at the synaptic terminals of the photoreceptors, ultimately transmitting the visual signal to bipolar cells and subsequently to ganglion cells in the retina.

Thus, the absorption of