Cyclic nucleotide-regulated channels C
Unless otherwise stated all data on this page refer to the human proteins. Gene information is provided for human (Hs), mouse (Mm) and rat (Rn).
Overview
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Cyclic nucleotide-gated (CNG) channels are responsible for signalling in the primary sensory cells of the vertebrate visual and olfactory systems. A standardised nomenclature for CNG channels has been proposed by the NC-IUPHAR subcommittee on voltage-gated ion channels [9].
CNG channels are voltage-independent cation channels formed as tetramers. Each subunit has 6TM, with the pore-forming domain between TM5 and TM6. CNG channels were first found in rod photoreceptors [7,10], where light signals through rhodopsin and transducin to stimulate phosphodiesterase and reduce intracellular cyclic GMP level. This results in a closure of CNG channels and a reduced ‘dark current’. Similar channels were found in the cilia of olfactory neurons [11] and the pineal gland [6]. The cyclic nucleotides bind to a domain in the C terminus of the subunit protein: other channels directly binding cyclic nucleotides include HCN, eag and certain plant potassium channels.
Hyperpolarisation-activated, cyclic nucleotide-gated (HCN)
The hyperpolarisation-activated, cyclic nucleotide-gated (HCN) channels are cation channels that are activated by hyperpolarisation at voltages negative to ~-50 mV. The cyclic nucleotides cyclic AMP and cyclic GMP directly activate the channels and shift the activation curves of HCN channels to more positive voltages, thereby enhancing channel activity. HCN channels underlie pacemaker currents found in many excitable cells including cardiac cells and neurons [5,12]. In native cells, these currents have a variety of names, such as Ih, Iq and If. The four known HCN channels have six transmembrane domains and form tetramers. It is believed that the channels can form heteromers with each other, as has been shown for HCN1 and HCN4 [1]. A standardised nomenclature for HCN channels has been proposed by the NC-IUPHAR subcommittee on voltage-gated ion channels [9].
Channels and Subunits
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CNGA1
C
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CNGA2
C
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CNGA3
C
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CNGB3
C
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HCN1
C
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HCN2
C
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HCN3
C
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HCN4
C
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Further reading
How to cite this family page
Database page citation:
Martin Biel, Elvir Becirovic, Verena Hammelmann, Franz Hofmann, U. Benjamin Kaupp. Cyclic nucleotide-regulated channels. Accessed on 24/06/2018. IUPHAR/BPS Guide to PHARMACOLOGY, http://www.guidetopharmacology.org/GRAC/FamilyDisplayForward?familyId=71.
Concise Guide to PHARMACOLOGY citation:
Alexander SPH, Striessnig J, Kelly E, Marrion NV, Peters JA, Faccenda E, Harding SD, Pawson AJ, Sharman JL, Southan C, Davies JA; CGTP Collaborators. (2017) The Concise Guide to PHARMACOLOGY 2017/18: Voltage-gated ion channels. Br J Pharmacol. 174 Suppl 1: S160-S194.
CNGA1, CNGA2 and CNGA3 express functional channels as homomers. Three additional subunits CNGA4 (Q8IV77), CNGB1 (Q14028) and CNGB3 (Q9NQW8) do not, and are referred to as auxiliary subunits. The subunit composition of the native channels is believed to be as follows. Rod: CNGA13/CNGB1a; Cone: CNGA32/CNGB32; Olfactory neurons: CNGA22/CNGA4/CNGB1b [13,18-21].
Hyperpolarisation-activated, cyclic nucleotide-gated (HCN)
HCN channels are permeable to both Na+ and K+ ions, with a Na+/K+ permeability ratio of about 0.2. Functionally, they differ from each other in terms of time constant of activation with HCN1 the fastest, HCN4 the slowest and HCN2 and HCN3 intermediate. The compounds ZD7288 [2] and ivabradine [3] have proven useful in identifying and studying functional HCN channels in native cells. Zatebradine and cilobradine are also useful blocking agents.