Nicotinic acetylcholine receptors cationic channels formed

Nicotinic acetylcholine receptors (nAChRs) are a family of ligand-gated ion channels that are found in abundance at the neuromuscular junction, and are widely distributed throughout the central and peripheral nervous systems. They comprise five subunit proteins that combine to form a cation permeable pore at the cell surface. In neuronal tissues, multiple nAChR subtypes may be assembled from a diverse array of subunits (a2-a9 32-p4) and their... 

A FIGURE 7-45 Three-dimensional structure of the nicotinic acetylcholine receptor, (a) Schematic cutaway model of the pentameric receptor in the membrane for clarity, the p subunit is not shown. Each subunit contains an M2 a helix (red) that faces the central pore. Aspartate and glutamate side chains at both ends of the M2 helices form two rings of negative charges that help exclude anions from and attract cations to the channel. The gate, which is opened by binding of acetylcholine, lies within the pore, (b) Top Cross section of the exoplasmic face of the... 

The well-studied nicotinic acetylcholine receptors of vertebrates usually consist of two a and three non-a subunits, but a9 forms a heteromer with alO with a stoichiometry of two a9 and three alO subunits when expressed in Xenopus oocytes [8]. Also, a7, a8, and a9 subunits can form functional homomers in the oocyte expression system [9-11], The cation-permeable ion channels of nAChRs open in response to binding of ACh. Most neonicotinoids are agonists of native... 

All cell membranes contain transmembrane proteins that form ion channels. These ion channels are usually selectively permeable to particular ions. Some channels, such as GABA-gated ion channels, are permeable to Cl ions and are inhibitory in nature because they make the inside of the nerve or muscle cells more negative as the Cl ions enter. Some ion channels are permeable to the cations Na and K, and an example of this type is the nicotinic acetylcholine-gated channel. Nicotinic channels have an excitatory effect when they open because Na ions enter and K ions leave through these channels. The cell becomes more positive inside and depolarizes. If the cell is a muscle cell, calcium accumulates in the cytoplasm and it contracts. We have found that all over the surface of Ascaris muscle there are GABA receptors (Martin, 1980) as well as nicotinic acetylcholine channels (Martin, 1982 Robertson and Martin, 1993). 

Nicotinic acetylcholine (ACh) receptors are responsible for transmission of nerve impulses from motor nerves to muscle fibers (muscle types) and for synaptic transmission in autonomic ganglia (neuronal types). They are also present in the brain, where they are presumed to be responsible for nicotine addiction, although little is known about their normal physiological function there. Nicotinic receptors form cation-selective ion channels. When a pulse of ACh is released at the nerve-muscle synapse, the channels in the postsynaptic membrane of the muscle cell open, and the initial electrochemical driving force is mainly for sodium ions to pass from the extracellular space into the interior of the cell. However, as the membrane depolarizes, the driving force increases for potassium ions to go in the opposite direction. Nicotinic channels (particularly some of the neuronal type) are also permeable to divalent cations, such as calcium. 

These characteristics are shared with subunits that form other ion channels/receptors and thus define a receptor superfamily, usually referred to as the nicotinic family. All members in this superfamily function as either cation- or anion-selective channels, thereby mediating fast excitatory or inhibitory synaptic transmission. In mammalian cells, the cation-selective members include nicotinic and 5HTg receptors, whereas the anion-selective members include GABA, GAB Ac, and glycine receptors. Anion-selective channels in this family are also found in invertebrates these channels are gated by glutamate, 5-HT, histidine, and acetylcholine (1). 

This can be activated by nicotine and for this reason is called a nicotinic receplor. The receptor-channel complex is pentameric and is constructed from four different protein subunits (aaPyE in the adult) that span the membrane and are arranged to form a central pore (channel) through which cations (mainly Na+) flow. Acetylcholine molecules bind to the two a-subunit.s inducing a confoimational change that opens tlic channel for about I millisecond. 

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