“…In the dark, the cyclic GMP level in the rod outer segments is high, and illumination causes a decrease in the level of cyclic GMP in the whole retina (11,25,29,35), in photoreceptor cells (36), and in isolated outer segments (21,49,50), apparently through activation of the degradation enzyme, cyclic GMP-phosphodiesterase (9,24,33,48) . In view of the evidence of these studies, cyclic GMP is considered to be one of the possible visual transmitters that could transfer information from the site of photon absorption in the disc membranes to the permeability mechanism in the plasma membrane (16,27).…”