Cyclic nucleotide-gated ion channel

A Cyclic nucleotide-gated ion channel is any ion channel that opens in the presence of cyclic nucleotides. The channels are gated by a chemical ligand (the cyclic nucleotide) but they are more similar in structure to the family of voltage-gated ion channels than to the ligand gated ones. In fact, the cyclic nucleotide-gated channels often have positively or negatively charged areas that may respond to changes in membrane potential. The purpose and function of these charged areas are not yet fully understood.

Cyclic nucleotide-gated channels are particularly important in the mammalian olfactory and visual systems. In the visual system, a cGMP (cyclic guanosine monophosphate) gated channel is found in the outer membrane of retinal photoreceptor cells. In response to high levels of cGMP, the channels are open and allow positively charged ions to flow into the cell, causing depolarization. This is the state of the cell in the dark (called the dark current), but a photon striking a photoreceptor in the cell causes a chain reaction that results in lower levels of cGMP and therefore hyperpolarization. Thus, these cells are actually more active in dark then light.