Excitation and adaptation in the vertebrate rod photoreceptor

The excitatory processes and the reactions of the rhodopsin photoproducts in vertebrate rods are summarized, and an adaptation model of the rod is outlined. The transmitter factor Q describes the ability of the discs to release a transmitter carrying the signal from the site of light adaptation to the plasma membrane of the receptor. If Q decreases appropriately when the background light intensity increases, the response range of the rod is preserved and the rod obeys Weber's law. This kind of receptor adaptation is experimentally observed in several species. A photoisomerization in a disc apparently affects the release of a transmitter also in neighbouring discs. The background light also affects the neuronal network of the retina, preserving the response range of the ganglion cell. After an exposure bleaching an appreciable fraction of rhodopsin the sensitivity of the rod returns in two phases: a fast intermediate adaptation and a slow opsin adaptation. After a rather small bleach the photoresponse is fairly well preserved (mainly Q-adaptation), but large bleaches cause a strong reduction in the response range. Different experimental methods utilized when correlating rhodopsin photoproducts and intermediate adaptation are discussed.