Cyclic nucleotide-gated ion channel



A cyclic nucleotide-gated (CNG) ion channel is any ion channel that opens in the presence of cyclic nucleotides.

Mechanism
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.

Functions
Cyclic nucleotide-gated channels are particularly important in several systems:


 * The mammalian olfactory system. It may also be involved in the insect olfactory system.


 * 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 than in light.


 * Funny currents have been associated with the HCN channels.


 * cGMP-gated cation channels are present in the inner medullary collecting ducts of the kidney. They promote Na+ reabsorption. A part of the atrial natriuretic peptide mechanism of action is inhibition of these channels, just as the diuretic drug amiloride. Aldosterone has an opposite effect.

Alpha
"Cyclic nucleotide gated channel alpha" subunits include
 * Cyclic nucleotide-gated channel alpha 1
 * Cyclic nucleotide-gated channel alpha 2
 * Cyclic nucleotide-gated channel alpha 3
 * Cyclic nucleotide-gated channel alpha 4

Beta-subunits include:
 * Cyclic nucleotide-gated channel beta 1
 * Cyclic nucleotide-gated channel beta 3

Hyperpolarization-activated
Other examples include the following "hyperpolarization-activated cyclic nucleotide-gated potassium channels" (HCN channels): HCN1, HCN2, HCN3, HCN4