Cyclic Nucleotide-Gated (CNG) channels belong to the superfamily of voltage-gated ion channels. Although permeable to various ions such as the monovalent Na⁺ and K⁺ ions, and the divalent Ca²⁺ ion, they are gated by the intracellular binding of the cyclic nucleotides cAMP and cGMP and not by voltage per se (CNGs bind preferably to cGMP)¹.

Six channels form this subfamily: The A subunit (CNGA1-4) and the B subunit (CNGB1 and CNGB3). Functional entities are formed by the assembly of four subunits. Each subunit consists of six membrane spanning domains, where the fourth transmembrane domain is highly positively charged, a pore domain between transmembrane domains five and six, and intracellular N- and C-termini. The cyclic nucleotide binding domain is located in the C-terminal region and is responsible for the channel gating upon cyclic nucleotide binding¹.

These channels are highly expressed in retinal photoreceptors and olfactory neurons where their role has been extensively studied². CNG channels have also been detected in brain, testis and kidney although their role in these tissues has yet to be unraveled².   

When heterologously expressed all subunits with exception of CNGA4, CNGB1 and CNGB3 can form functional homomeric channels. In olfactory sensory neurons, CNG channels which are formed by two CNGA2, one CNGA4 subunits and an isoform of CNGB1, are activated by the binding of cAMP, whose levels increase in response to the binding of odorant molecules to their respective receptors^1,3.

CNGA4 knockout mice are fertile and morphologically similar to their wild type littermates^4,5. In these mice, the olfactory bulb and epithelium are developed but these mice display defects in odor adaptation, and at the molecular level, they are less sensitive to cAMP activation⁶.