Cyclic nucleotide-gated (CNG) channels are nonselective cation channels first identified in retinal photoreceptors and olfactory sensory neurons (OSNs). They are opened by the direct binding of cyclic nucleotides, cAMP and cGMP. Although their activity shows very little voltage dependence, CNG channels belong to the superfamily of voltage-gated ion channels. Like their cousins the voltage-gated K+ channels, CNG channels form heterotetrameric complexes consisting of two or three different types of subunits. Six different genes encoding CNG channels, four A subunits (A1 to A4) and two B subunits (B1 and B3), give rise to three different channels in rod and cone photoreceptors and in OSNs. Important functional features of these channels, i.e., ligand sensitivity and selectivity, ion permeation, and gating, are determined by the subunit composition of the respective channel complex. The function of CNG channels has been firmly established in retinal photoreceptors and in OSNs. Studies on their presence in other sensory and nonsensory cells have produced mixed results, and their purported roles in neuronal pathfinding or synaptic plasticity are not as well understood as their role in sensory neurons. Similarly, the function of invertebrate homologs found in Caenorhabditis elegans, Drosophila, and Limulus is largely unknown, except for two subunits of C. elegans that play a role in chemosensation. CNG channels are nonselective cation channels that do not discriminate well between alkali ions and even pass divalent cations, in particular Ca2+. Ca2+ entry through CNG channels is important for both excitation and adaptation of sensory cells. CNG channel activity is modulated by Ca2+/calmodulin and by phosphorylation. Other factors may also be involved in channel regulation. Mutations in CNG channel genes give rise to retinal degeneration and color blindness. In particular, mutations in the A and B subunits of the CNG channel expressed in human cones cause various forms of complete and incomplete achromatopsia.

Cyclic Nucleotide-Gated Ion Channels

Nitric oxide (NO) has only recently been appreciated as a normal biologic substance with a role in signal transduction. It was first identified as endothe­ lial-derived relaxing factor in blood vessels and as the mediator of the tumor­ icidal and bactericidal actions of macrophages. NO's role as a neural messenger may be even more prominent. Biosynthesis of NO involves oxidation of the guanidine group of arginine with stoichiometric formation of citrulline. NO synthase is one of the most extensively regulated enzymes in biology. In the periphery, NO is a likely transmitter of nonadrenergic, noncholinergic neurons. In the brain, NO neurons mediate action of glutamate acting at N-methyl-D­ aspartate (NMDA) receptors. Excess release of NO appears to account for a major portion of neural damage following vascular stroke.

Nitric Oxide in the Nervous System

https://www.cell.com/neuron/pdf/S0896-6273(00)80201-9.pdf

A putative cyclic nucleotide-gated channel is required for sensory development and function in C. elegans.

Cyclic nucleotide phosphodiesterases regulate cAMP-mediated signaling by controlling intracellular cAMP content. The cAMP-hydrolyzing activity of several families of cyclic nucleotide phosphodiesterases found in human heart is regulated by cGMP. In the case of PDE2, this regulation primarily involves the allosteric stimulation of cAMP hydrolysis by cGMP. For PDE3, cGMP acts as a competitive inhibitor of cAMP hydrolysis. Several cGMP-mediated responses in cardiac cells, including a potentiation of Ca(2+) currents and a diminution of the responsiveness to beta-adrenergic receptor agonists, have been shown to result from the effects of cGMP on cAMP hydrolysis. These effects appear to be dependent on the specific spatial distribution of the cGMP-generating and cAMP-hydrolyzing proteins, as well as on the intracellular concentrations of the two cyclic nucleotides. Gaining a more precise understanding of how these cross-talk mechanisms are individually regulated and coordinated is an important direction for future research.

cAMP and cGMP Signaling Cross-Talk: Role of Phosphodiesterases and Implications for Cardiac Pathophysiology

An ion channel directly activated by cGMP was first discovered about ten years ago. Since then, a number of ion channels with the same property (cyclic nucleotide-activated channels) have been reported that are involved in a variety of cell functions. In addition, other channels have been found that are not primarily controlled by cyclic nucleotides but are modulated by them (cyclic nucleotide-modulated channels). These channels likewise have diverse functions and tissue distributions. Both channel classes are reviewed here. Coverage includes the cyclic-nucleotide binding site on these channels, ion permeation, pharmacological blockers, channel gating and modulation, and physiological functions of the channels.

CYCLIC NUCLEOTIDE-GATED ION CHANNELS: An Extended Family With Diverse Functions

Cyclic nucleotide-gated cation channels are essential in visual and olfactory signal transduction An additional member of the cGMP-gated channel family, termed CNG-3, has been cloned from bovine kidney Its deduced amino acid sequence is 60% and 62% identical with the CNG-channel proteins from bovine rod outer segment and bovine olfactory epithelium, respectively Northern analysis and sequences amplified by the PCR showed that the CNG-3 mRNA is present in testis, kidney, and heart Calcium permeated the expressed channel in the presence of extracellular Mg2+ and Na+ at membrane potentials from -100 to +45 mV It is likely that CNG-3 protein is responsible for cGMP-induced Ca2+ entry in cells other than sensory cells

Another member of the cyclic nucleotide-gated channel family, expressed in testis, kidney, and heart

Expression of cyclic nucleotide-gated cation channels in non-sensory tissues and cells.

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Another member of the cyclic nucleotide-gated channel family, expressed in testis, kidney, and heart

Expression of cyclic nucleotide-gated cation channels in non-sensory tissues and cells.