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.

In the oviduct, cumulus cells that surround the oocyte release progesterone. In human sperm, progesterone stimulates a Ca(2+) increase by a non-genomic mechanism. The Ca(2+) signal has been proposed to control chemotaxis, hyperactivation and acrosomal exocytosis of sperm. However, the underlying signalling mechanism has remained mysterious. Here we show that progesterone activates the sperm-specific, pH-sensitive CatSper Ca(2+) channel. We found that both progesterone and alkaline pH stimulate a rapid Ca(2+) influx with almost no latency, incompatible with a signalling pathway involving metabotropic receptors and second messengers. The Ca(2+) signals evoked by alkaline pH and progesterone are inhibited by the Ca(v) channel blockers NNC 55-0396 and mibefradil. Patch-clamp recordings from sperm reveal an alkaline-activated current carried by mono- and divalent ions that exhibits all the hallmarks of sperm-specific CatSper Ca(2+) channels. Progesterone substantially enhances the CatSper current. The alkaline- and progesterone-activated CatSper current is inhibited by both drugs. Our results resolve a long-standing controversy over the non-genomic progesterone signalling. In human sperm, either the CatSper channel itself or an associated protein serves as the non-genomic progesterone receptor. The identification of CatSper channel blockers will greatly facilitate the study of Ca(2+) signalling in sperm and help to define further the physiological role of progesterone and CatSper.

Sea urchin eggs attract sperm through chemotactic peptides, which evoke complex changes in membrane voltage and in the concentrations of cyclic AMP, cyclic GMP and Ca2+ ions The intracellular signalling pathways and their cellular targets are largely unknown. We have now cloned, from sea urchin testis, the complementary DNA encoding a channel polypeptide, SPIH. Functional expression of SPIH gives rise to weakly K+-selective hyperpolarization-activated channels, whose activity is enhanced by the direct action of cAMP. Thus, SPIH is under the dual control of voltage and cAMP. The SPIH channel, which is confined to the sperm flagellum, may be involved in the control of flagellar beating. SPIH currents exhibit all the hallmarks of hyperpolarization-activated currents (Ih), which participate in the rhythmic firing of central neurons, control pacemaking in the heart, and curtail saturation by bright light in retinal photoreceptors. Because of their sequence and functional properties, Ih channels form a class of their own within the superfamily of voltage-gated and cyclic-nucleotide-gated channels.

Ionic currents activated by hyperpolarization and regulated by cyclic nucleotides were first discovered more than 20 years ago. Recently the molecular identity of the underlying channels has been unveiled. The structural features of the protein sequences are discussed and related to the mechanisms of activation, selectivity for cyclic nucleotides, and ion permeation. Coverage includes a comparison of the biophysical properties of recombinant and native channels and their significance for the physiological functions of these channels.

Sensory transduction in vertebrate photoreceptors and olfactory sensory neurons is mediated by cyclic nucleotide-gated (CNG) channels that conduct mono- and divalent cations. Ca2+ entering the cell through CNG channels intimately controls signaling pathways by regulating several key enzymes. Cloned CNG channels from photoreceptors and olfactory sensory neurons profoundly differ in their relative Ca2+ permeability, their blockage by external divalent cations, and the fraction of current carried by Ca2+. In particular, CNG channels from cone photoreceptors conduct significantly more Ca2+ than those from rod photoreceptors. Furthermore, the current through the olfactory CNG channel is entirely carried by Ca2+ at approximately 3 mM extracellular Ca2+. These results suggest that a major function of CNG channels is to provide a pathway for Ca2+ entry.