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The signal transduction cascade responsible for sensing light in vertebrates is one of the best studied signal transduction processes, and is initiated by rhodopsin in rod cells, a member of the G-protein coupled receptor gene family. Rhodopsin remains the only GPCR whose structure has been resolved at high resolution. Rhodopsin in the discs of rod cells contains a bound 11-cis retinal chromophore, a small molecule derived from Vitamin A that acts as the light sensitive portion of the receptor molecule, absorbing light to initiate the signal transduction cascade. When light strikes 11-cis retinal and is absorbed, it isomerizes to all-trans retinal, changing the shape of the molecule and the receptor it is bound to. This change in rhodopsin\'s shape alters its interaction with transducin, the member of the G-protein gene family that is specific in its role in visual signal transduction. Activation of transducin causes its alpha subunit to dissociate from the trimer and exchange bound GDP for GTP, activating in turn a membrane-bound cyclic-GMP specific phosphodiesterase that hydrolyzes cGMP. In the resting rod cell, high levels of cGMP associate with a cyclic-GMP gated sodium channel in the plasma membrane, keeping the channels open and the membrane of the resting rod cells depolarized. This is distinct from synaptic generation of action potentials, in which stimulation induces opening of sodium channels and depolarization. When cGMP gated channels in rod cells open, both sodium and calcium ions enter the cell, hyperpolarizing the membrane and initiating the electrochemical impulse responsible for conveying the signal from the sensory neuron to the CNS. The rod cell in the resting state releases high levels of the inhibitory neurotransmitter glutamate, while the release of glutamate is repressed by the hyperpolarization in the presence of light to trigger a downstream action potential by ganglion cells that convey signals to the brain. The calcium which enters the cell also activates GCAP, which activates guanylate cyclase (GC-1 and GC-2) to rapidly produce more cGMP, ending the hyperpolarization and returning the cell to its resting depolarized state. A protein called recoverin helps mediate the inactivation of the signal transduction cascade, returning rhodopsin to its preactivated state, along with the rhodopsin kinase Grk1. Phosphorylation of rhodopsin by Grkl causes arrestin to bind, helping to terminate the receptor activation signal. Dissociation and reassociation of retinal, dephosphorylation of rhodopsin, and release of arrestin all return rhodopsin to its ready state, prepared once again to respond to light. (This definition may be outdated - see the DesignNote.)
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