As light intensity increases, the amount of cGMP in photoreceptor cells typically...

As light intensity increases, the amount of cyclic guanosine monophosphate (cGMP) in photoreceptor cells decreases. In the dark, high levels of cGMP are present, which keeps the cGMP-gated ion channels open, allowing sodium ions to enter the cell and resulting in a depolarized state. When light strikes the photoreceptors, it activates the photopigment rhodopsin, which in turn activates a signaling cascade involving phosphodiesterase (PDE). This enzyme reduces the levels of cGMP in the cell. As cGMP levels drop, the cGMP-gated channels close, leading to hyperpolarization of the photoreceptor cells.

This hyperpolarization is crucial for the process of visual transduction, as it reduces the release of neurotransmitter and allows for the transmission of signals to bipolar cells in the retina. Thus, the decrease in cGMP corresponding to increased light intensity is a fundamental part of the mechanism by which photoreceptors adapt to varying light conditions.