Plasma membrane Disc membrane R* R -G G -GTP PDE G PDE OL, E GDP photon Na GTP 0GDP GMP GTP Figure 5. Visual excitation pathway in rod photoreceptors. The initial events of phototrans duction occur on the physically separate disc membranes in the outer segment portion of the cell. Pho toactivation of rhodopsin (R) catalyzes the activation of hundreds of heterotrimeric G-proteins (G) Nucleotide exchange on the G-subunit is accelerated, and the dissociated G -GTP subunit then inter acts with the PDE6 holoenzyme (PDE). Displacement of the PDE6 subunit from the catalytic site by G GTPrelieves the inhibition of catalysis at one catalytic subunit, resulting in rapid hydrolysis of cy toplasmic cGMP. The drop in cGMP levels in the outer segment causes dissociation of cGMPfrom the cGMP-gated ion channels in the plasma membrane, causing their closure. The reduced entry of cations into the outer segment causes membrane hyperpolo arization, and ultimately, generation of the receptor potential at the photoreceptor synapse. Reactions involved in the recovery of the photoresponse and desensitization of the light response are not shown