can be achieved by differential control of the affinity for binding of the ligand, efficacy of gating, or a combination of both. binding affinity means how tightly cyclic nucleotides bind to the channel. efficacy refers to the ability of ligand to activate and open the channel once it is bound. although these processes are useful in understanding selectivity, they are inextricably coupled to each other that it is very difficult to experimentally separate one from another. cng channels do not discriminate between na+ and k+ ions, and they also allow ca2+ and mg2+ to pass through, although at a slower rate. the passage of these divalent ions inhibit the current carried by na+ and k+. a highly conserved residue of glutamic acid in the selectivity filter of cng channels has been found to form a high-affinity binding site for ca2+. moreover, a bacterial nonselective cation channel called the nak channel hosts a selectivity filter sequence similar to that of cng channels. in the crystal structure of the nak channel, a discrete ca2+-binding site at the extracellular opening of the pore has been identified. inhibition of cng channels studies have shown the differential inhibition of cng channels by diacylglycerol (dag) in xenopus oocytes. dag can be used as a closed state inhibitor. using dag to inhibit homomultimeric rod channels, or those that contain two or more identical peptide chains, was similar to the inhibition of native rod channels. using dag to inhibit the homomultimeric olfactory channels was not as effective, even with a high concentration of dag. more than one molecule of dag is needed to inhibit a channel effectively. dag molecules position themselves to stabilize the close state of a cng channel, by binding to the channel or by changing the interaction between the lipid bilayer of the cell membrane and the channel. studies with chimeras rod and olfactory channels suggest that differences in dag inhibition are due to differences in the attached loop of the transmembrane segment. physiological significance photoreceptors main article: photoreceptor cell in the absence of light, cgmp binds to cng channels in photoreceptors. this binding causes the channels to open, which allows sodium (na+) and calcium (ca2+) ions to flow into the cell causing the outer segment of the photoreceptor to depolarize. this depolarizing flow of ions is known as the dark current. when the retina of the eye detects light, a reaction known as a phototransduction cascade occurs. it is a signal transduction pathway that leads to the activation of the enzyme phosphodiesterase, which hydrolyzes cgmp into 5’-gmp, decreasing the concentration of cgmp. in the absence of cgmp, the cng channels in the photoreceptors close preventing the flow of the aforementioned dark current. this in turn causes a hyperpolarization of the outer segment of the photoreceptor, preventing the propagation of an action potential and the release of glutamate. studies have shown that over activation of cgmp-dependent cng channels in photoreceptors can lead to their degeneration. if the cng channels on a photoreceptor are continuously activated, ca2+ and na+ ion flux into the outer segment of the photoreceptor will increase so that it depolarizes beyond the dark current. through a positive feedback loop, this would then increase the current of ca2+ into the cell. high concentration of ca2+ in the photoreceptor cell would lead to its death programmed cell death or apoptosis. retinitis pigmentosa main article: retinitis pigmentosa fundus of patient with retinitis pigmentosa, mid stage (bone spicule-shaped pigment deposits are present in the mid periphery along with retinal atrophy, while the macula is preserved although with a peripheral ring of depigmentation. retinal vessels are attenuated.) retinitis pigmentosa (rp) is a genetic disease in which patients suffer degeneration of rod and cone photoreceptors. the loss starts in the patient's peripheral vision and progresses to the central visual field, leaving the patient blind by middle age. about 1% of rp patients have mutations in cgmp alpha-subunit. eight mutations have been identified- four are nonsense mutations, one is a deletion that includes most of the transcriptional unit. the other three are missense mutations and frameshift mutations, which lead to a shortening of amino acid sequence in the c terminus. it is still not known why the absence of cgmp-gated cation channels causes photoreceptor degradation. mutations causing rp have also been found in the rhodopsin gene and in the alpha- and beta-subunits of rod phosphodiesterase, which encode rod phototransduction cascades. the mutation of these subunits indirectly impairs rod cgmp-gated channel function, which implies that there is a common mechanism of photoreceptor degradation. pacemaker cells main article: hcn channel in the nervous system, heart, and some visceral organs, cells contain cyclic nucleotide gated channels which determine the rhythm of the organ. these channels, formally called hyperpolarization-activated cyclic nucleotide–gated channels (hcn channels), are also termed "pacemaker channels" because of this critical function. as their name implies, they are open during conditions of hyperpolarization and closed during depolarization. the significance of this in the sinoatrial node (and, as backup, in the atrioventricular node) is that as the heart resets, or hyperpolarizes, after each beat, hcn channels open, allowing positive ions to rush into the cell (the so-called funny current), triggering another depolarization event and subsequent cardiac contraction. this gives the heart its automaticity. the primary cyclic nucleotide operating in conjunction with the hcn channel is camp. olfactory sensory neurons almost all responses to odorants in olfactory sensory neurons (osns) are facilitated by cng channels. when an odorant binds to its specific receptor in the chemosensitive cilia membrane, it activates a g protein, which causes a downstream reaction activating the enzyme adenylyl cyclase (ac). this enzyme is responsible for an increase in camp concentration within the osn. camp binds to the cng channels in the osn membrane, opening them, and making the cell highly permeable to ca2+. calcium ions flow into the cell causing a depolarization. as in all other cell types, cng channels in osns also allow na+ to flow into the cell. additionally, the increased ca2+ concentration inside the cell activates ca2+-dependent chloride (cl−) channels, which causes intracellular cl− ions to also flow out of the cell augmenting the depolarization event. this depolarization stimulates an action potential that ultimately signals the reception of the odorant. in addition to camp gated ion channels, a small subset of osns also has cgmp-selective cng channels that contain the cnga3 subunit. spermatozoa main article: spermatozoon camp and cgmp mediate several cellular responses such as acrosomal exocytosis, or the fusion of the male sperm to the female egg, and chemotaxis. in the sea urchin species, strongylocentrotus purpuratus, speract, a short peptide, was studied. speract activates a receptor-type guanylate cyclase (gc) and stimulates a rise in intracellular cgmp concentrations. speract also increases the concentration of calcium. although there has yet to be any establishment of a direct causal relationship, the previously mentioned observations suggest that cgmp activates calcium conductance. cng channels are prime candidates for the calcium-entry pathway, due to their high calcium permeability. cng channels have yet to be detected by homology screening. in mammals, the testicular cng channel subunits that are expressed are a3, b1, and b3. heterologous expression of the a3 subunit was cloned from testis and produced channels that were cgmp sensitive and selective. it is possible that these channels are involved in a cgmp-stimulated calcium flux into the sperm. however a more extensive characterization of the channel has not been accomplished due to the low success rate of detecting channel activity. since a3 subunit knockout mice are fertile, cng channels could be involved in some form of motility control and even in chemotactic swimming behavior or in the acrosomal exocytosis. however, a receptor-type gc in mammalian sperm has yet to be identified. mouse sperm express other channels such as catsper1. male sterility can be accomplished by disrupting the catsper1 gene; additionally, the camp-induced calcium influx is abolished in mutant mice. although catsper need additional subunits to become functional, they are unrelated to cng channels because catsper lacks a camp/cgmp-binding site. it is possible that cng and catsper subunits assemble to form calcium-permeable and cyclic nucleotide-sensitive ion channels. kidney cgmp-sensitive channels have been analyzed in the renal medulla, specifically in collecting duct cells, which influence the body's electrolyte and fluid balance. cng channel activity is controlled by the interaction between cgmp-dependent protein kinase and g1 protein because of cgmp's involvement in phosphorylation mechanisms. in the cells from an inner medullary collecting duct, cng channels exhibit cation selectivity unit conductance, calcium permeability, and pharmacology very similar to cyclic nucleotide-gated ion channels. a stimulant, atrial natriuretic factor (anf) increases cgmp production in the kidneys, which increases function of the glomerulus by a combination of relaxing and contracting arterioles. differences between retinal and renal cdna have been implicated in the functional differences between cng channels in these two tissues. gonadotropin-releasing hormone there has been identification of cng ion channel subunits a2, a4, and b1 in a neuronal cell line that secretes gonadotropin-releasing hormone (grh). the three subunits make up the cng channels on chemosensitive cilia of osns. in high extracellular calcium, the unit conductance of cng channels in rods and osns are significantly smaller than those measured in the neuronal line. it seems doubtful that cng channels would create large unit conductance. cryoem structure of a prokaryotic cyclic nucleotide-gated ion channel. plants cng ions channels in plants are similar in amino acid sequence and structure to non-selective cation cng channels in animals, as well as trans-membrane-domain k+-selective shaker family channels. however, there are drastic differences that are seen exclusively in plant cng channels. the amino acid sequence of the pore sequence in plant cng channels lacks the selectivity filter found in animal cng channels as well as lacks a glycine-tyrosine-glycine-aspartate (gygd) motif in the k+-selectivity filter sequence. other sequence differences are seen in plant cng channels, particularly in the cyclic nucleotide binding domain (cnbd). in plants, the calmodulin binding domain (cambd) is found to overlap α-helix c in the cnbd of cng channels. in animals the cambds are located far away from the cnbd. cng channels play a large role in plant immunity and response to pathogens or external infectious agents. they have also been implicated in apoptosis in plants. cng ion channels are also thought to be involved in pollen development in plants, however its exact role in this mechanism is still not known. unlike animal cng channels, plant cng channels have not been extensively analyzed biochemically with respect to their structure. prokaryotes cng ion channels share a high degree of sequence and structural similarity to mammalian cng channels. like mammalian cng channels, binding of cyclic nucleotides to the cnbds has been shown to regulate channel activity and alter the channel conformational state. because these channels were only recently identified in spirochaeta and leptospira species, their precise physiological function remains unknown in these organisms. in combination with photoactivated adenylyl cyclases, they have been used as optogenetic tools to inhibit action potential generation in neurons. current and future research researchers have answered many important questions regarding cng ion channels functions in vision and olfaction. in other physiological areas, the role of cng channels is less defined. with technological growth, there now exists more possibilities for understanding these mechanisms. because nitric oxide (no) is involved in stimulating the synthesis of cgmp, further research is being conducted to understand the physiological interaction of no with cng channels, particularly in the covalent modification of cng channels in osns. scientists are adding on to the mechanism involved in the interaction of binding sites and interfaces of subunits. this might be nonexistent in non-cooperative cng channels. it is also possible that binding site and gate are attached to a single subunit. in order to develop these ideas, double electron-electron resonance (deer) and rapid fixing techniques can show these mechanistic movements. a 2007 study suggests that because of the various and complex regulatory properties in addition to the large number of cng channels in plants, a multidisciplinary study to research plant cng channels should be conducted. another study in march 2011 recognizes recent reverse genetics data that has been helpful in further understanding cng channels in plants, and also suggests that additional research be conducted to identify the upstream and downstream factors in cngc-mediated signal transduction in plants. scientist are speculating whether dag directly binds with cng channel during inhibition.